Animal Diversity: Chordata

List Of Contents 

I. Introduction 

Animal Diversity: Chordata 

Anamniotes 

Amphibia 

Dr. Monisha Khanna 

Acharya Narendra Dev College 

University of Delhi 

Kalkaji 

New Delhi – 110 019 

Monisha_khanna@hotmail.com 

II. Classification 

A. Order 1. Apoda (Gymnophiona / Caecilia) 

B. Order 2. Urodela (Caudata) 

C. Order 3. Anura 

III. Origin 

A. From Early Chordates To The First Land Vertebrates 

a. Origin of Chordates 

b. Origin of Vertebrates 

c. Origin of Tetrapods 

B. Acquisition of Adaptations For Life On Land 

IV. Amphibia: General Organization 

a. External Appearance 

b. Integument 

c. Alimentary Canal 

d. Respiratory Organs And Voice Apparatus 

e. Blood Vascular System 

f. Endoskeleton 

g. Nervous System And Sensory Organs 

h. Urinogenital System And Osmoregulation 

V. Parental Care In Amphibia 

A. Order Anura 

B. Order Urodela 

C. Order Apoda

I. INTRODUCTION 

Members of the phylum Chordata are commonly referred to as chordates. Four characters, of 

prime diagnostic importance, are possessed by all chordates: 1) A primitive endoskeletal 

structure called the notochord is present during early embryonic life. This pliant, rod-like 

structure, composed of a peculiar type of connective tissue, is located along the mid-dorsal 

line, where it forms the axis of support for the body. In some animals it persists as such 

throughout life, but in most chordates it serves as a foundation around which the vertebral 

column is built. 2) A hollow, dorsal nerve tube is present sometime during life. The central 

nervous system, made up of the brain and the spinal cord, is located in a dorsal position just 

above the notochord. It is a hollow canal from one end to the other. 3) Gill slits or traces of 

them connecting to the pharynx are present at some stage of life. Most aquatic chordates 

respire by gills made up of vascular lamellae or filaments lining the borders of the gill slits, 

which connect to the pharynx and open directly or indirectly to the outside. Even terrestrial 

chordates, which never breathe by gills, nevertheless have traces of gill slits present as 

transient structures, during early embryonic life. No vascular lamellae line these temporary 

structures, nor do they open to the outside, but the fact that they are present in all chordates is 

of prime importance in denoting close relationship. 4) Chordates possess a post-anal tail in 

some stage of their life that represents a posterior elongation of the body extending beyond 

the anus. The tail is primarily an extension of the chordate locomotor apparatus, the 

segmental musculature and notochord. Apart from the above four features, chordates also 

have certain characteristics common to some other phyla as well. 5) They are bilaterally 

symmetrical; 6) are metameric; 7) have a true body cavity or coelom, lined with mesoderm; 

8) show cephalization or the concentration of nervous tissue and specialized sense organs in 

or towards the head; 9) the blood is pumped anteriorly from the ventrally located heart and 

forced to the dorsal side. It then moves posteriorly and returns to the heart by veins. 

The phylum Chordata is usually subdivided into four main groups or subphyla. The first three 

of these include a few relatively simple animals, which lack a cranium and brain. These 

organisms are sometimes collectively referred to as the Acrania. The animals included in this 

category are believed to show similarities to the chordate ancestors, hence are frequently 

known as the protochordates. These are the subphyla Hemichordata (acorn worms), 

Urochordata (tunicates), and the Cephalochordata (amphioxus). Vertebrata (Craniata) is a 

large group, embracing chordates having a brain; endoskeleton; notochord not extending 

forward under the brain; paired eyes; presence of red blood cells; a ventrally placed heart; 

presence of a sympathetic nervous system; and presence of a hepatic portal system. 

Vertebrates include the jawless forms, lacking vertebrae (Super class Agnatha), and the jawed 

vertebrates (Super class Gnathostomata). Furthermore, the latter include the series, Pisces 

embracing the lower forms commonly known as fishes. The remaining vertebrates are 

included in the group Tetrapoda, which are basically four-footed animals, although in some 

the limbs have been lost or modified secondarily. 

Tetrapods are those members of the subphylum Vertebrata having paired appendages in the 

form of limbs rather than fins, though in some forms the limbs either degenerate completely 

or show modifications. Among other characteristics which distinguish tetrapods from fishes 

are a cornified outer layer of skin; nasal passages which communicate with the mouth cavity 

and which transport air; lungs used in respiration; and a bony skeleton along with a reduction 

in the number of skull bones. 

2

Tetrapoda 

The class Amphibia is composed of tetrapods in which the transition from aquatic to 

terrestrial life is clearly indicated. Amphibians are the first vertebrates to live on land, 

although they lay their eggs in water or in moist situations. 

The first tetrapods evolved from rhipidistian crossopterygian fishes. The fossil remains of 

primitive tetrapods have been found in the eastern parts of Greenland in Devonian deposits. 

These specimens have features intermediate between late crossopterygians and early 

amphibians. 

The group Tetrapoda is divided into four classes made up of amphibians, reptiles, birds and 

mammals. The living representatives of the class Amphibia include salamanders, newts, 

frogs, toads, and the caecilians. The amphibians lead a double life, that is, first in the water, 

and then on the land. The result of this ambitious attempt is that they present a medley of 

makeshift adaptations, which leave them still a long way from vertebrate perfection. 

Among the dual adjustments that they make, are those associated with locomotion and 

protection against desiccation. In water, an elongated fishlike body, propelled by a muscular 

tail, has proved to be the most efficient mechanism for locomotion. However on land, the 

weight of the body is no longer supported by the surrounding aqueous medium, so that the 

two pairs of appendages become modified into legs, which act as levers to lift the body away 

from the ground. Such levers are equipped with adequate muscles without adding excessively 

to the body weight. However the amphibians are not particularly successful at locomotion on 

land. Even in frogs and toads, where amphibian legs reach their highest development, such 

locomotor appendages are so inefficiently anchored to a single vertebra of the supporting 

backbone that these animals cannot bear their weight upon them in the sustained manner 

necessary for standing or walking, and can progress only by the momentary exertion of 

hopping or jumping. 

The problem of dessication arises from the fact that the surrounding air, takes up moisture 

rapidly from any moist surface. Amphibians not only utilize gills and primitive lungs in 

respiration, but also exchange gases to a very large extent directly through the skin. 

Consequently, these animals can live only in moist places. In comparison, the higher land 

animals, in which an efficient pulmonary system is formed, are not restricted because they 

develop a thick, relatively dry integument, which is resistant to dessication. Thus, relatively 

3

inefficient respiratory organs, together with other anatomical handicaps prevent amphibians 

from maintaining a body temperature independent of that of the surroundings. 

The difficulty of avoiding dessication is also involved in the breeding habits of amphibians 

because they have not made the changes required of true land vertebrates. No amnion (liquidfilled 

sac) is produced by the embryos of lower vertebrates including the Amphibia. The 

latter must therefore go back to the water to breed in most cases. Furthermore, the 

metamorphosis of such an amphibian as a frog or a toad, necessitated by its emergence from 

water to land, works profound changes both in its structure and in its feeding habits. For 

instance, during its lifetime a toad changes its diet six times. While in the egg it absorbs the 

yolk; upon hatching it develops a temporary mouth and eats the jelly of the egg envelopes; 

next it becomes the free swimming tadpole feeding mainly upon the aquatic vegetation; the 

juvenile stage has fat bodies provided to meet the intervening demands of hibernation; with 

the warmth of spring the young toad catches slugs and insects for a living. 

The distinct features of amphibians can be summarized as follows: 

1. Amphibians are ectothermal vertebrates. 

2. They have varied body forms – ranging from elongated forms, with a distinct head, 

trunk and tail; to a compact, depressed body with a fused head and trunk and no 

intervening neck. 

3. Limbs are usually four in number, although some forms are limbless. 

4. Skin is smooth and moist with many glands including pigment cells. Poison glands are 

sometimes present but scales are mostly absent. 

5. Mouth is usually large, with small teeth in either upper or both jaws. Teeth are bicuspid 

and pedicellate. In some forms, teeth are completely absent. The nostrils open into the 

anterior part of the mouth cavity. 

6. Skeleton is mostly bony, with varying number of vertebrae; ribs are present in some 

forms but absent in others. Ribs if present do not encircle the body. Centra of vertebrae 

are cylindrical. Similar type of vertebra is also found among several groups of early 

tetrapods. There is the presence of double or paired occipital condyle. The posterior 

skull bones have been lost. Small, widely separated pterygoids are found. A small bone 

in the skull called operculum is present and is fused to the ear bones in most anurans; it 

is perhaps involved in hearing and balancing. 

7. Ability to elevate the eye with specially developed levitator bulbi muscle. There is also 

the presence of a special type of visual cell in the retina known as the green rod. (This 

however is absent in Apoda). 

8. Respiration occurs by lungs, skin and gills, either separately or in combination. A 

forced pump respiratory mechanism exists. The larval forms have the external gills that 

may persist throughout life in some forms. 

9. Presence of a three-chambered heart having two atria and one ventricle. A double 

circulation takes place through the heart. 

10. The excretory system consists of paired mesonephric kidneys and urea is the main 

nitrogenous waste. 

11. Sexes are separate; fertilization is mostly internal in salamanders and caecilians but 

generally external in frogs and toads. Amphibians are predominantly oviparous, rarely 

ovoviviparous. Eggs are moderately yolky with jelly-like membrane coverings. 

Metamorphosis is usually present. Fat bodies are associated with gonads. 

4

II. CLASSIFICATION 

Basal tetrapods have been variably subdivided although the relationships among these groups 

remain unclear. The most primitive amphibians known from fossil remains are the 

Labyrinthodonts dating back to the late Devonian period, the name being based upon the 

complex folding of the enamel layer of the teeth. These animals are sometimes called the 

Stegocephalians because of the solid roofing of the skull. In certain features they resembled 

the rhipidistian crossopterygian fishes. Many labyrinthodonts had an armor of overlapping 

bony plates. By the Carboniferous, many groups are recognized including Temnospondyli, 

Anthracosauria, and Microsauria. It is not clear as to what is the relationship of living 

amphibians known as Lissamphibia to these groups. One hypothesis suggests that 

lissamphibians are the sister group to Temnospondyli. Alternatively, lissamphibians may 

have evolved from a temnospondyl ancestor. It is hypothesized that lissamphibians are either 

monophyletic (a common temnospondyl ancestor) or diphyletic (apodans descended from a 

microsaurian ancestor). What we can say from the knowledge of amphibian relationships is 

that the class Amphibia, as traditionally defined, is a paraphyletic group that omits its 

amniote descendants. Successive mutations and natural selection increasingly adapted basal 

amphibian descendants for terrestrial life culminating with the origin of the amniotes. 

The following classification is as given by Young: 

Table 1: Classification of amphibia 

Subclass 1: 

Labyrinthodontia (folded teeth) 

Order 1: 

Ichthyostegalia (fish vertebrae) 

eg.: Ichthyostega, Elpistostege 

Order 2: 

Temnospondyli (divided 

vertebrae). 

Suborder 1: 

Rhachitomi (stem animals) 

eg.: Loxomma, Eryops, Cacops, 

Archegosaurus 

Suborder 2: 

Stereospondyli (ring vertebrae) 

eg.: Capitosaurus, Buettnaria, 

Mastodonsaurus 

Order 3: 

Anthracosauria (coal lizards). 

Eg.: Palaeogyrinus, Seymouria, 

Pteroplax 

Subclass 2: 

Lepospondyli (scale 

vertebrae) 

eg.: Diplocaulus, 

Ophiderpeton, 

Microbrachis, Sauropleura 

5 

Subclass 3: 

Lissamphibia (smooth 

amphibia) 

Order 1: 

Urodela / Caudata (tails) 

eg.: Molge, Salamandra, 

Triton, Ambystoma, 

Necturus 

Order 2: 

Apoda / Caecilia / 

Gymnophiona (no limbs). 

eg.:Ichthyophis, 

Typhlonectes 

Order 3: 

Anura (no tails) 

Eg.:Protobatrachus, 

Leiopelma, Rana, Bufo, 

Hyla, Pipa

The following alternate classification of amphibians is as given by Parker and Haswell: 

Table 2: Classification of amphibia 

Subclass 1: 

Apsidospondyli 

Super order 1: 

Labyrinthodontia 

Order 1: 

Ichthyostegalia 

Order 2: 

Rhachitomi 

Order 3: 

Stereospondyli 

Order 4: 

Embolomeri 

Order 5: 

Seymouriamorpha 

Super order 2: 

Salientia 

Order 1: 

Eoanura 

Order 2: 

Proanura 

Order 3: 

Anura 

Subclass 2: 

Lepospondyli 

Order 1: 

Aistopoda 

Order 2: 

Nectridia 

Order 3: 

Microsauria 

Order 4: 

Urodela 

Order 5: 

Apoda 

Labyrinthodonts The oldest amphibians were the swamp-dwelling labyrinthodonts. 

Ichthyostega was the earliest specimen appearing in the Devonian. Labyrinthodonts were a 

large, widely dispersed and diverse assemblage. On the basis of the morphology of their 

vertebrae, paleontologists have been of the opinion that fossil amphibians with 

stereospondylous and embolomerous vertebrae were not in the amniote line. Labyrinthodonts 

had many features seldom seen in modern amphibians. These included minute bony scales in 

the skin dermis; a fishlike tail supported by dermal fin rays; and skull similar to those of 

rhipidistian fishes. Labyrinthodonts, like their aquatic ancestors, had a sensory canal system 

of neuromast organs. One or another of the labyrinthodonts was ancestral to the first amniote. 

Temnospondyls was a group that was common in the Permian with its fossil record 

extending back to the Mississippian. Members of the temnospondyls have achieved skeletal 

similarities to modern frogs and salamanders, suggestive of their close relationship. A 

number of lissamphibian skeletal features and their relatively smaller size can be explained as 

the retention of juvenile ancestral temnospondyl features. The condition in caecilians does 

not fit easily into this scenario, possibly suggesting an independent origin from microsaurs. 

6

Microsaurs represent a diverse group of fossil forms known from the Pennsylvanian to the 

lower Permian. They share a number of skeletal features with caecilians, which may suggest 

either a close relationship or convergence on an elongate body form specialized for 

burrowing. 

Anthracosaurs Anthracosauria is a small Paleozoic group, thought to be in direct line to the 

amniotes. Their fossil record extends from the Mississippian to the Triassic. 

Lissamphibians Living amphibians of approximately 2000 species may be grouped in three 

orders: Apoda, Urodela and Anura. 

a. ORDER 1. APODA (GYMNOPHIONA / CAECILIA). 

Members of the order are pantropical in distribution. The caecilians are burrowing forms, 

with worm like bodies, lacking limbs. The tail is very short suited to their mostly 

terrestrial habits and the anus is almost terminal. The skull is solid and bony, again suited 

for a burrowing lifestyle. The animals are blind, but carry special sensory tentacles. 

Unlike other amphibians, some caecilians have dermal scales. Adults lack gills and gill 

slits. The very small eyes are buried beneath the skin or under the skull bones. Because of 

the presence of an intromittent organ in males, internal fertilization is assumed. In some 

caecilians, eggs are laid, which hatch into free-living larvae. The eggs are large, yolky 

and cleavage is meroblastic; they are laid on land in Ichthyophis, and the embryos 

develop around the yolk sac, but often have long, plumed gills. The female guards the 

eggs until the larvae hatch and move to the aquatic habitat. Other genera skip over the 

aquatic larval stage and a few have specialized external gills. In still other genera, the 

eggs are retained within the female, metamorphosis occurring before birth. Viviparity is 

common in the aquatic form, Typhlonectes. Important Apoda families are as follows: 

7

Table 3: Classification of Apoda up to families 

Family Character 

Eocaecilia 

Rhinotrematidae Small (up to 30 cm) 

Terrestrial with aquatic 

larvae 

Ichthyophidae Moderately large (up to 50 

cm) Terrestrial with aquatic 

larvae 

Uraeotyphlidae Small terrestrial, oviparous 

forms with possibly direct 

development 

Scoleocomorphidae Moderately large terrestrial 

forms, possibly viviparous 

Caeciliidae Very small (10 cm) to very 

large (1.5 cm) terrestrial 

and aquatic forms, 

oviparous and viviparous 

species, no aquatic larval 

stage 

Typhlonectidae Small to large (75 cm) 

aquatic and semi-aquatic 

forms, viviparous with 

aquatic larvae 

b. ORDER 2. URODELA (CAUDATA). 

Distribution Example 

Fossil from Early 

Jurassic of N. 

America 

9 Species in S. 

America 

Eocaecilia 

Epicrionops, 

Rhinatrema 

36 Species in Asia Caudacaecilia, 

Ichthyophis 

4 Species in India Uraeotyphlus 

5 Species in Africa Crotaphatrema, 

Scoleocomorphus 

~ 90 Species in 

Central and S. 

America, Africa, 

India and the 

Seychelles Islands 


America 

Boulengerula, 

Brasilotyphlus, 

Caecilia, 

Dermophis, 

Gegeneophis, 

Geotrypetes, 

Gymnopis 

Typhlonectes, 

Atretochoana, 

Chthonerpeton, 

Nectocaecilia, 

Potomotyphlus 

These include the salamanders and newts, the latter being small, semi-aquatic forms. 

Urodeles are found in temperate and subtropical climates in the Northern Hemisphere but 

do not reach the tropics in the New World. The elongated body consists of head, trunk, 

and a well developed tail, the latter being retained throughout life. Two pairs of limbs 

occur in most species. Larvae resemble adults except for the presence of gills, and like 

adults, have teeth in both the upper and lower jaws. The urodeles have a greater tendency 

to show generalized characters of the class amphibia, in comparison to the much more 

specialized Anura. The group shows different types of forms, varying from the terrestrial 

salamanders, such as Salamandra maculosa, which is viviparous, to the fully aquatic 

forms, such as Necturus. Furthermore, there is a tendency to retain larval characters in the 

adults of certain aquatic forms, the process known as paedomorphosis / neoteny. 

Examples include Megalobatrachus, which has no eyelids but loses its gills in the adult; 

In Cryptobranchus, the spiracle remains open being used for expulsion of water during 

8

espiration. Amphiuma is an elongated form with very small legs, no eyelids and four 

branchial arches. An extreme example of neotenous forms is Necturus, which has external 

gills but has such a reduced lung that the animal can live as a permanently aquatic form. 

Similarly, Siren shows all larval characters and has no hind limbs. The terrestrial newts 

are of different types: some are definitely terrestrial like Triturus vulgaris, although it is 

not able to live in very dry habitats. The limbs support the body weight, their soles being 

applied to the ground and turned forwards. The tail shows reduction to form a rod-like 

organ but when the animal returns to the water for breeding purposes, the tail develops a 

large fin. On the other hand, is the genus Ambystoma, which has eleven species, in which 

some races become mature without metamorphosis, because of lack of iodine in water, 

whereas others are genetically neotenous. The important families of urodeles include: 

Table 4: Classification of Urodela up to families 


Distribution Example Species 

Karaurus Fossil from Jurassic of 

Kazakhstan 

Sirenidae Small (15 cm) to large 

(75 cm) elongate 

aquatic forms, with 

external gills, pelvic 

girdles and hind-limbs 

absent 

Cryptobranchidae Very large (1 m) to 

huge (> 1.5 m) aquatic 

forms, paedomorphic 

with external 

fertilization of the eggs 

Hynobiidae Small to medium size 

(30 cm) aquatic or 

terrestrial forms, 

external fertilization of 

eggs, aquatic larvae 

Amphiumidae Very large (1m) 

elongate, aquatic 

forms, lacking gills 

Plethodontidae Tiny (3 cm) to large 

(30 cm) aquatic or 

terrestrial forms, direct 

development or some 

with aquatic forms 

9 

4 Species in N. 

America 


America and 2 Species 

in Asia 

Karaurus sharovi 

Siren, Habrosaurus, 

Pseudobranchus 

Cryptobranchus, 

Megalobatrachus 

~ 36 Species in Asia Batrachuperus, 

Hynobius, 

Onychodactylus, 

Pachynynobius, 

Ranodon, 


America 

~ 265 Species in N., C. 

and S. America, 1 

Species in Europe 

Salamendrella 

Amphiuma 

Gyrinophilus, Eurycea, 

Pseudotriton, 

Manculus


RHYACOTRITONIDAE Very small (

latter living on land have a warty dry skin, and shorter hind limbs for hopping. Anurans 

inhabit a wide variety of habitats, ranging from arid deserts to mountainous regions to 

swampy areas to tropical rain forests. Temperature and water regulation are critical to 

amphibians generally, and the anurans particularly. Being ectothermal, frogs and toads 

depend on the ambient temperature for body temperature regulation. In winters, frogs in 

temperate zones hibernate or enter into a state of extremely reduced activity. On the other 

hand, they avoid the extreme heat of summer months in the tropics, by remaining 

underground during daytime and being active at night. Anurans are also susceptible to the 

loss of body moisture due to extremely hot or dry conditions. Those in temperate climates 

maintain moist skin to assist in evaporative cooling. In addition, their permeable skin, 

gives the frog an ability to absorb water simply by jumping into water. In contrast are the 

frogs in arid regions, which have the skin impermeable to water so as to prevent rapid 

evaporation and dehydration. Instead, they cover their body with a mucus film, or burrow 

to avoid the heat altogether. 

Breeding in frogs is triggered by temperature change and rainfall. During the breeding 

season, thousands of frogs may congregate. The males attract their mates by calling. The 

latter usually occurs near a water body, where the eggs can be laid and fertilized. Parental 

care is variable; some species lay many smaller eggs and show no parental care, while 

others lay a few larger eggs and remain with them till the young ones develop. 

Among the frogs and toads, many genera are suited for special modes of life. Ascaphus 

and Leiopelma, for example live in mountain streams and have reduced lungs. These 

show a combination of specialized and primitive features. Internal fertilization occurs by 

a penis-like extension of the cloaca. The primitive characters include: presence of tail 

muscles, amphicoelous vertebrae, free ribs, abdominal ribs, and persistent posterior 

cardinal veins. In Alytes, the males carry the eggs wrapped around the legs. The related 

aquatic frog, Pipa, is still more specialized, having no tongue and having developed an 

elaborate arrangement by which the young are carried in pits on the back. Xenopus is 

related to Pipa, but without the habit of carrying its young. The bufonid toads are among 

the most successful of all amphibian groups and well adapted for a terrestrial life, though 

always returning to the water to breed. Bufo and related genera are cosmopolitan in 

distribution. Only one genus: Nectophrynoides is viviparous. Hyla and other tree frogs are 

similar to the bufonids, but show many arboreal adaptations including the presence of 

pads on the toes for climbing. Many tropical frogs have devised methods of avoiding 

having to return to water for breeding. In Nototrema, for example, the young develop in a 

sac on the back of the female, this sac sometimes being protected by special calcareous 

plates. Rana and its allies, the true frogs, are also cosmopolitan. A number of its related 

genera have got adapted to an arboreal existence. An example is Polypedates, a 

widespread genus and several others, each independently derived from ranids. Burrowing 

forms have also developed among the anurans, as Breviceps, which digs for ants and has 

a snout, as in anteaters. The important anuran families are as follows: 

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Table 5: Classification of Anura up to families 

Family 

Triadobatrachus 

Character 

Ascaphidae Small (3 cm) aquatic 

forms, found in cold 

springs and 

mountain streams, 

fertilization internal 

Leiopelmatidae Small semi-aquatic 

or terrestrial forms 

Bombinatoridae 


Small (10 cm) Fossil from Early 

Triassic of Madagascar 

Small to medium 

size semi-aquatic 

forms 

DISCOGLOSSIDAE Small to medium 

size terrestrial and 

semi-aquatic forms 

Pipidae Specialized aquatic 

forms, direct 

development or with 


Rhinophrynidae Burrowing form 

with aquatic larvae 

Megophryidae Small to medium 

size forest-floor 

forms 

Pelodytidae Small terrestrial 

frogs with aquatic 

larvae 

Pelobatidae Short-legged 

terrestrial forms, 

with aquatic larvae 

Allophrynidae Small arboreal forms 

12 


America 

3 Species in New 

Zealand 

8 Species in Europe 

and Asia 

5 Species in Western 

Europe and North 

Africa 


America and Africa 

1 Species, extreme 

southern Texas to 

Costa Rica 

~ 80 Species from 

Pakistan, N. India 

through Southeast Asia 

and the Philippines to 

Indonesia 

2 Species in Europe 

and Asia 


America 

1 Species in S. America 

Triadobatrachus 

massinoti 

Ascaphus 

Leiopelma 

Bombina 

Discoglossus 

Hymenochirus, 

Pipa 

Rhinophrynus 

Megophrys 

Pelodytes 

Pelobates 

Allophryne

Family 

Character 

BRACHYCEPHALIDAE Very small ( 130 Species in 

Central and South 

America 

5 Species in extreme 

southern Africa 

~ 760 Species in N. C. 

and S. America, 

Europe, Asia and 

Australia 

>900 Species in 

southern N. America, 

Central and S. America 

and the West Indies 


Australia, Tasmania 

and New Guinea 

3 Species in the 

Seychelles Islands 

Brachycephalus 

Bufo 

Hyalinobatrachium 

Heleophryne 

Litoria, Hyla 

Eleutherodactylus 

Mixophyes 

Sooglossus

Family 

Character 

carried on the back 

of the adult 

PSEUDIDAE Aquatic frogs with 

huge 

tadpoles that 

metamorphose 

into medium-size 

adults 

Rhinodermatidae Small terrestrial 

frogs, 

tadpoles either 

transported to 

water or 

development 

completed in the 

vocal sacs of the 

male 

Arthroleptidae Small or medium 

size terrestrial frogs 

DENDROBATIDAE Small terrestrial 

frogs, brightly 

colored and highly 

toxic, tadpoles are 

transported to water 

by the adult, direct 

development 

Hemisotidae Small burrowing 

forms 

Hyperoliidae Small to medium 

sized, mostly 

arboreal frogs with 


Microhylidae Small to medium 

sized, terrestrial or 

arboreal frogs; 


mostly, some have 

non feeding 

tadpoles, others with 

direct development 

Ranidae Medium sized to 

enormous, aquatic or 

terrestrial frogs, 

aquatic tadpoles 

mostly, some with 

direct development 

14 


4 Species in S. America 

2 Species in southern 

Chile and Argentina 

75 Species from sub- 

Saharan Africa 

~185 Species in C. and 

S. America 

8 Species from sub- 

Saharan Africa 


Africa, Madagascar, 

and the Seychelles 

Islands 

~315 Species in N. C. 

and S. America, Asia, 

Africa and Madagascar 

> 700 Species in N. C. 

and S. America, 

Europe, Asia and 

Africa 

Lysapsus, Pseudis 

Rhinoderma 

Arthroleptis 

Dendrobates 

Hemisus 

Leptopelis 

Hypopachus 

Lithobates, Rana

Family 

Character 

Rhacophoridae Very small to large, 

mostly arboreal 

frogs, filter feeding 

aquatic larvae, some 

lay eggs in tree holes 

and have non 

feeding larvae 

Scaphiopodidae Round with short 

legs, Terrestrial 


>900 Species in 

southern N. America, 

Central and S. America 

and the West Indies 

Native to Southern 

Canada and U.S.A 

South to Southern 

Mexico, comprising of 

seven families 

Amphignathodontidae Native to Neotropical 

America (=Central 

America and South 

America) 

Mantellidae Terrestrial, 

arboreal or 

aquatic. Body size 

ranges from 3 to 10 

cm in length 

III. ORIGIN 

A. From early chordates to the first land vertebrates 

Found only in 

Madagascar and 

Mayotte 

Rhacophorus 

Spea 

Gastrotheca, 

Flectonotus, 

Amphignathodon 

Mantella, 

Laliostoma, 

Aglyptodactylus, 

Wakea, 

Blommersia, 

Guibemantis 

The vertebrate story unfolds over a span of almost 544 million years, during which time; 

some of the largest and most complex animals ever known have evolved among the 

vertebrates (Figure 1). They show all the four defining chordate characters: notochord, 

pharyngeal slits, tubular dorsal nerve cord and a post-anal tail. Vertebrates occupy marine, 

freshwater, terrestrial, and aerial environments and exhibit a vast array of lifestyles. The 

oldest craniates include the vertebrate fossils from the Lower Cambrian of China. These are 

the ostracoderms. These strange fishes, 2 cm to 2 m long and of diverse appearances, had no 

jaws, most were without paired fins, and were filter feeders. Broad bony plates in the skin 

formed a protective shield over the head and trunk. Fossils that can be clearly identified as 

ostracoderms date back to the beginning of the Ordovician. The jawless ostracoderms were 

succeeded in the seas by jawed fishes, and amphibians eventually became established on 

land. The perplexing problem is who were the ancestors of ostracoderms? 

15

a. Origin of Chordates 

Ostracoderms were chordates, therefore we can look for clues in the protochordates that are 

with us today and in the fossil record that immediately preceded ostracoderms to trace the 

ancestry of ostracoderms. Cephalochordates have a notochord; pharyngeal slits; a dorsal 

hollow central nervous system with brain and cord; a metameric body wall musculature; a 

two-layered skin; and arterial and venous channels similar to those of fishes and to the 

embryonic vessels of tetrapods. Cephalochordates are deuterostomous, coelomate, and filter 

feeders, as were many early ostracoderms. These similarities bespeak close genetic ties 

between the ancestors of cephalochordates and those of vertebrates. Although we know on 

the basis of the current data, that protochordates preceded craniates in the course of natural 

history, we have to speculate concerning the lineages that might have led from prechordate 

invertebrates, to protochordates on the one hand, and to craniates on the other. 

Cephalochordates as we know them today were not the genetic ancestors of the first 

craniates. We must consider the observation that echinoderms, like vertebrates have 

16

mineralized tissue in their mesoderm; that echinoderms, like cephalochordates, form their 

mesoderm and coelom as outpocketings of their archenteron; that echinoderms, 

enteropneusts, cephalochordates, and craniates are all deuterostomes a trait found in only one 

other invertebrate taxon, the Chaetognatha; and that all have larvae in their history. 

Ongoing phylogenetic research and the availability of new molecular methods provide an 

improved, although certainly incomplete view of protochordate evolution. Vertebrates arise 

within the deuterostome radiation, part of the chordate clade (Figure 2). The other clade 

includes the echinoderms along with the hemichordates, which are more closely related to 

each other than to the chordates. Some fossil echinoderms preserved a bilateral symmetry, 

but most, including all living groups, diverged dramatically, becoming pentaradial losing 

pharyngeal slits and a distinct neurulated nerve cord. Hemichordates are monophyletic, with 

pterobranchs arising within the enteropneusts, and retain some chordate characters 

(pharyngeal slits, neurulated nerve cord, and endostyle). Urochordates are also monophyletic, 

a sister group to the rest of the chordates (cephalochordates plus vertebrates). 

Cephalochordates are the immediate relatives of the vertebrates. 

This phylogenetic view suggests that a wormlike ancestor, perhaps similar to an enteropneust 

worm, evolved into the hemichordates / echinoderms on one side of the deuterostomes and 

into a chordate on the other. Strictly speaking, this means that chordates did not evolve from 

echinoderms and certainly not from annelids / arthropods. Although unsettled and 

controversial in its specifics, the origin of chordates lies certainly somewhere among the 

invertebrates, a transition occurring in the remote Proterozoic times. Within the early 

chordates the basic body plan was established: namely, pharyngeal slits, notochord, dorsal 

hollow nerve cord, and the post anal tail. Feeding depended on the separation of suspended 

food particles from the water and involved the pharynx, a specialized area of the gut with 

walls lined by cilia to conduct the flow of food-bearing water. Pharyngeal slits allowed a one- 

17

way flow of water. Locomotor equipment included a notochord and segmentally arranged 

muscles extending from the body into a post anal tail. Subsequent evolutionary modifications 

were centered on feeding and locomotion and led to the wealth of adaptations found within 

the later vertebrates. 

b. Origin of Vertebrates 

The origin and early evolution of vertebrates took place in marine waters. Evolution of early 

vertebrates was characterized by increasingly active lifestyles hypothesized to proceed in 

three major steps: Step 1 comprised a suspension-feeding Prevertebrate, which deployed 

only cilia to produce the food-bearing current. Step 2 comprised an Agnathan, an early 

vertebrate lacking jaws but possessing a muscular pump to generate a food-bearing current. 

Step 3 comprised a Gnathostome, a vertebrate with jaws. It fed on larger food items with a 

muscularized mouth that rapidly snatched prey from the water. 

Conodonts are fossils extremely common in rocks from the late Cambrian to the end of the 

Triassic. The fossils bore evidence that the conodonts are vertebrates. The trunk showed 

evidence of V-shaped myomeres, a notochord down the midline, and caudal fin rays on a 

post-anal tail. There was also the presence of mineralized dental tissues: cellular bone, 

calcium phosphate crystals, calcified cartilage, enamel and dentine. The conodont feeding 

apparatus consisted of tongue-like or cartilaginous plates that moved in and out of the mouth, 

catching and delivering the crushed food. Thus this mechanism is very similar to the lingual 

feeding mechanism of hagfishes. Following the conodonts, Ostracoderms appeared in the 

very late Cambrian and radiated in the Silurian and early Devonian. Like the conodonts, they 

had complex eye muscles and dentine-like tissues. They were the first vertebrates to possess 

paired appendages, a lateral line system, an inner ear with two semicircular canals, and bone 

although the latter is located in the outer exoskeleton that encases the body in a bony armor 

just beneath the epidermis. 

One of the most significant changes during early vertebrate evolution was the development of 

jaws in primitive fishes, derived from the anterior pharyngeal arches. Two early groups of 

jawed fishes are known: The Acanthodians and the Placodermi. This adaptation opened up 

an expanded predatory way of life. Early gnathostomes also had two sets of paired fins, the 

pectorals and the pelvics, that were articulated with supportive bony or cartilaginous girdles 

within the body wall. This radiation of gnathostomes proceeded along two major lines of 

evolution: one produced the Chondrichthyes, the other the Teleostomi. The modern 

chondrichthyans consist of two groups: the sharks and rays (elasmobranchs) and the 

chimaeras (holocephalans). Both groups have similar fin structures, cartilaginous skeleton 

and pelvic claspers. The Teleostomi is a large group embracing the acanthodians, the bony 

fishes, and their tetrapod derivatives. Most living vertebrates are bony fishes, members of the 

Osteichthyes. Bony fishes have a set of characters including an adjustable, gas-filled swim 

bladder (possibly modified from lungs) to provide buoyancy; and an extensive ossification of 

the endoskeleton. Bony fishes consist of two unequal-sized groups, the actinopterygians that 

compose the vast majority of bony fishes; and the sarcopterygians. The latter group is 

important as they gave rise to the very first terrestrial vertebrates. The group called 

rhipidistians includes the sarcopterygians that are most closely related to tetrapods. Ossified 

neural and hemal arches accompany the notochord. The braincase had a hinge-like joint 

running transversely across its middle so that the front of the braincase swiveled on the back 

of the braincase. Simultaneously, there were modifications in skull bones and jaw 

musculature, bringing about a specialized feeding style involving a powerful bite. The jaws 

had labyrinthodont teeth characterized by complex infolding of the tooth wall around a 

central pulp cavity. Rhipidistians gave rise to tetrapods during the Devonian but they became 

extinct in the early Permian. The demands of terrestrial life and the new opportunities 

18

available led to an extensive remodeling of the fish design as tetrapods diversified into 

terrestrial and eventually aerial modes of life. It however, also includes some derived groups 

with secondary loss of limbs, such as snakes. 

c. Origin of Tetrapods 

The development of vertebrates which lived on land, started about 350 million years ago in 

the Devonian period. At this time, some fish began to crawl out of the water and started 

walking on land and breathing air. The climate of the world at the end of Devonian became 

hot and arid. This would have caused the water in shallow pools and lakes to become warmer, 

and many small water bodies may have evaporated during the seasonal droughts. The fish 

ancestors of the first land vertebrates must have had two important features i.e. firstly: the 

presence of lungs as simple pouches leading from the throat, which developed a rich supply 

of blood vessels. And secondly: the development of limbs from the bony supports of the fins. 

The most likely ancestors of the amphibians were the rhipidistians that were common in the 

Permian. Unfortunately, the fossil record of the origin of amphibians is very poor. Rock 

deposits from the middle Devonian period contain typical rhipidistian fish; while early 

amphibian ancestors appear in the late Devonian. However no fossil species, which directly 

link the two groups, have been found during the intervening period of about 30 million years. 

Until more fossil species are found, which show the transitional forms between fishes and 

amphibians, this important period of vertebrate evolution will remain uncertain. The earliest 

fossil amphibians that have been found had already solved the problems of living on land. 

They were the Labyrinthodontia, and Ichthyostega is a typical example. The earliest 

amphibians were all carnivores and must have been feeding on other animals. Modern 

amphibians are specialized animals, which do not resemble primitive amphibians very 

closely. They are so specialized that it is not clear when they separated from the primitive 

amphibia, from which group they derived, or how closely related the modern forms are to 

each other. Three distinct groups of modern amphibia remain, which as adults feed on insects 

or other small invertebrates. These groups clubbed together as Lissamphibia include: 

Urodela (newts and salamanders); Anura (frogs and toads); and Gymnophiona (caecilians). 

B. Acquisition of adaptations for life on land 

Amphibia, the first vertebrates to become adapted to a terrestrial mode of life, may be 

differentiated from their fish predecessors, mainly on the basis of their pentadactyl limbs; the 

absence of fin rays in the unpaired fins, if present; and by the presence of a middle ear. 

Amphibians breathe by gills in the larval stages and by lungs when adult. The skin, which is 

usually naked, often plays an important role in respiration. The skull is autostylic and the free 

hyomandibular has got converted into a columella auris that stretches between the inner ear 

and the tympanic membrane. An opening called fenestra ovalis is present through which the 

columella transmits sound vibrations to the inner ear. This is another important modification 

with respect to terrestrial mode of life. 

Important changes have also occurred in the skeleton and musculature: The skull has become 

movably attached to the vertebral column by one or two occipital condyles. The head no 

longer supported by water, required a more powerful musculature and corresponding 

elaboration of articular surfaces in the skull and adjacent endoskeleton. The lower jaws too, 

required an elaborate musculature for their support and operation. The girdles had not only to 

provide support in locomotion but also to protect the internal organs from injury. With the 

advent of heavy upward pressures during walking, there arose anteriorly a powerful scapula 

(shoulder blade) bound to the front ribs of the thorax; and posteriorly a triradiated pelvic 

apparatus. In each girdle, there also arose endoskeletal processes for the firmer attachment of 

muscles and the developed specialized limb bones, including digits and other refinements. 

19

The complex system of tetrapod limb muscles is arranged in two series that are derived from 

the simpler musculature of the upper and lower aspects of fins. A comparison of the bony and 

skeletal structures of crossopterygians and early amphibians shows that their limbs are very 

closely allied. 

Ancient tetrapods, the Labyrinthodonts retained bony scales in the abdominal region. 

Grooves in the skull of some juveniles carried the lateral line system, which however was 

absent in the adults of the same species. Thus, many ancient tetrapods, like modern 

amphibians, were probably aquatic as juveniles and terrestrial as adults. These 

labyrinthodonts / stegocephalia originated no later than the Upper Devonian. Their important 

features include: 1) the loss of bones rigidly linking the skull to the shoulder girdle. This was 

accompanied by the appearance of a mobile neck allowing the head to be moved relative to 

the trunk. 2) The operculum was lost, as it was no longer needed in these early choanates 

since they had lost the internal gills of their early ancestors. 3) A reduction of the notochord 

and a rigid spine. The thick centra constricted the notochord. Special articulatory surfaces 

called as zygapophyses linked the neural arches to each other. Also the notochord was shorter 

and did not extend into the braincase. A sacral rib connected the axial skeleton to the pelvic 

girdle, allowing the weight of the tetrapod body to be transmitted to the hind limb. The 

dermal fin rays that were no longer needed on land were lost. 4) There was the development 

of four muscular limbs with discreet digits. 

The conquest of land also depended on some means of aerial respiration. The primitive lung 

is a characteristic of ancient fishes; it came first and from it evolved the swim bladder, an 

organ of specialized hydrostatic and other functions found only in bony fishes. In the 

Devonian, the development of a respiratory sac, capable of absorbing atmospheric oxygen, 

would be of immense help to early fishes that were compelled to live in water that became 

periodically low on oxygen level and clogged with rotting vegetation. The amphibians 

suffered a loss of true biting teeth. They were forced by virtue of their imperfect adaptation, 

to confine their feeding to slow moving prey and later to insects that could be reached with a 

sudden flick of the muscular, sticky and protrusible tongue that they later developed. The 

lateral line system, although retained in the larval forms, was soon lost in the land-living 

adults. 

Terrestrial life depended not only on the development of efficient lungs and walking legs, but 

every part of the animal body was involved. Not only did animals require to breathe 

atmospheric air and to walk; they had to withstand desiccation, rid themselves of the lateral 

line system, detect air-borne substances of a much greater dilution, see and hear predators and 

prey at much greater distances. However, no new organs were formed. 

The Devonian was an age of great climatic instability. The fresh water streams and lagoons of 

that time were alternately filled and dried out. Such conditions of periodic desiccation would 

have resulted in the extinction of numerous species, and resulted in the survival of those 

possessing the physiological and structural adaptations suitable to function in the new 

conditions. Ancient bony freshwater fishes had developed a bony supporting skeleton, 

osmotic homeostasis, internal nares, and lungs. In the crossopterygians, the appearance of 

two pairs of highly mobile, muscular, lobe-like lateral fins supported by bones, gave an 

indication of later development of walking legs of the modern tetrapods. This is a classic 

example of Pre-Adaptation: the possession by an organism of characters that are conducive to 

its survival under altered conditions. Thus those types that had locomotory, respiratory, 

integumentary, excretory and sensory specializations related to drought survival, would 

prosper and reproduce. Those whose adaptations were directed towards purely aquatic 

efficiency would fail. A comparison of the earliest Amphibia with Palaeozoic fishes shows 

many similarities between the embolomerous labyrinthodonts and the osteolepids of the 

20

Devonian. This is particularly marked in the general structure of the skulls, in the similar 

labyrinthodont pattern of the teeth, in the possession of the large palatal tusks, and in the 

structures of the girdles. 

The amphibian skin became heavily keratinized. This assisted water retention, but at the same 

time allowed cutaneous respiration. Yet, amphibians in general cannot live away from moist 

situations. The permeability of the amphibian skin has thus imposed serious limitations on the 

choice of habitat as well as upon geographical distribution. Secondly, the inevitable failure of 

the amphibia to develop temperature control further limits their chances of land colonization. 

From the time of their emergence they have remained imperfectly adapted to terrestrial life: 

most land-going species remained dependent upon fresh water for reproduction. Majority of 

amphibians require a damp environment in which to breed because their eggs and embryos 

must extract oxygen and food from the surrounding water and at the same time excrete waste 

material directly into it. They have developed no protective shell, as have reptiles, birds and 

primitive mammals. They also lay down little yolk for the nourishment of the growing young. 

The earliest known anuran ancestor, Protobatrachus, did not appear until the early Mesozoic. 

Although it had a tail in the adult, but the skull was similar to those of modern frogs. The ribs 

were short, the presacral vertebrae were reduced in number and there were free caudal 

vertebrae. There was an elongate ilium and a fairly long femur. However, the radius and ulna, 

and the tibia and fibula were separate. The earliest true frog remains occur in the Upper 

Jurassic. The earliest known salientia is Triadobatrachus massinoti, from early Triassic. This 

“proto-frog” is around 250 million years old and does not have the combination of characters 

normally associated with frogs. The earliest true frog is Vieraella herbsti from early Jurassic, 

while the fossil from middle Jurassic is Notobatrachus degiustoi, which is around 155-170 

million years old. Another well-preserved Jurassic fossil known as Eodiscoglossus santonje, 

has 8 pre-sacral vertebrae and thus is clearly within Anura. The Urodeles, too, have not been 

found before the Jurassic period. No fossil Apoda has been discovered. Today, the most 

terrestrial amphibia are the Anura. The Urodela have retreated once more to the water and the 

degenerate Apoda into moist holes on the ground. Although amphibians were the dominant 

land fauna in the Carboniferous, little more than 2000 species live today, although some 

remain plentiful in appropriate areas. Of the three surviving amphibian groups, the Anura are 

abundant in all the greater zoogeographical regions but are absent from most oceanic islands. 

The Urodela are almost exclusively Palaearctic and Nearctic forms, occurring in North 

America, Europe, Asia, and North Africa. A few species extend southwards into the 

Neotropical and Oriental regions. The Apoda, on the other hand, are mainly tropical, 

occurring in the Neotropical, Ethiopian and Oriental regions. They are absent from 

Madagascar, Australasia, and the Pacific Islands. 

IV. AMPHIBIA: GENERAL ORGANIZATION 

a. External Appearance- In the Anura, the head is large and depressed with a wide 

mouth and large tympanic membranes in most genera. The eyes are large and 

prominent, provided with an upper eyelid and a nictitating membrane. The trunk is 

short, the tail absent and the cloacal aperture is terminal. The hind limbs are much 

longer than the forelimbs (Figure 3a). The forelimb consists of an upper arm or 

brachium, a forearm or antebrachium, and a hand or manus, ending in four tapering 

digits. The hind limb consists of the thigh or femur, the shank or crus, and the foot, 

which consists of a tarsal region and five slender digits. The arboreal forms have 

plate-like adhesive discs at the termination of the digits of all four legs. The discs hold 

on to the substratum with the help of capillary action and adhesion by means of 

21

mucus. In certain tree frogs, like Rhacophorus, the ventral surface and webbing of the 

elongated digits together produce a volplane mechanism functionally comparable with 

that of the reptile Draco, and certain marsupials and rodents. In the Apoda, the body is 

elongated and snake-like, the head is small and not depressed, and limbs are absent. Tail 

is absent. These tropical animals live in burrows, are blind although they possess sensory 

tentacles lodged in pits. In burrowing forms the snout is sharp and may have dermal 

ossifications. The Urodela are of three types: the perennibranchiate or persistent-gilled 

have an elongate trunk separated by a slight constriction from the depressed head and at 

the other end passes into a compressed tail having a continuous median fin. Limbs are 

small and weak and eyes are small and lidless. Tympanic membrane is absent. On each 

side of the neck are two-gill slits leading into the pharynx. From the dorsal end of each of 

the three branchial arches arises a branched external gill. To this group belong genera 

such as Siren, Necturus and Proteus. The remaining urodeles are called caducibranchiate 

or deciduous-gilled and are further of two types: the derotrematous forms, that loose the 

gills but retain the gill clefts in the adult, an example being Amphiuma whose body is eel 

like and the limbs are extremely small. To the second group belong the salamandrine 

forms, in which all trace of branchiate organization disappears in the adult as seen in the 

Spotted Salamander and the common British newts. The limbs in land salamanders stand 

out from the trunk and are plantigrade. 

22

The median fin is completely lost and the tail is cylindrical (Figure 3b). 

b. Integument- The epidermis of amphibians is composed of several layers of cells and 

is the first to have a dead stratum corneum. The epidermis is six to eight celled thick 

and divisible into three layers: stratum corneum, stratum germinativum, and a basal 

part in contact with the basement membrane. Shedding of dead skin fragments occurs 

periodically and consists of removing a unicellular sheet of stratum corneum. A dead 

corneal layer is an adaptation to terrestrial life, protecting the body and preventing 

excessive loss of moisture. The dermis is relatively thin in amphibians. It is composed 

of two layers: an outer stratum spongiosum; and an inner stratum compactum. A great 

number of gland types are recognized. Mucus glands are numerous; their secretion 

helps to maintain a moist skin. A second group of glands, the poison glands, secrete a 

number of substances that are distasteful and / or poisonous in some tropical frogs. 

The amphibian skin is an important organ of respiration. Blood vessels, lymph spaces, 

glands, and nerves are abundant in the stratum spongiosum while the inner stratum 

compactum is rich in muscle fibers. Chromatophores of complex structure lie between 

the epidermis and dermis (Figure 4). 

The skin is soft and usually slimy due to the secretion of the cutaneous glands. Many 

toads and a few salamanders have small poison glands aggregated into prominent 

swellings called the parotoid glands located on each side of the head. Furthermore, the 

large and conspicuous warts of toads are each perforated by a pore that leads to a 

23

poison gland beneath. The exudate from the gland contains toxins known as bufotalin 

and bufogin. Some specialized integumentary glands in amphibians have a tubular 

structure. They include those on the feet of certain tree-dwelling frogs and toads. 

Suctorial discs on the toes aid in climbing. Other examples include the glandular 

thumb pads of male frogs and toads and the mental glands of the male salamanders. 

Unicellular glands are present on the snouts of tadpoles and urodele larvae. Their 

secretion has digestive properties, thus helping in freeing the larva from the egg 

capsule during early development. Large glands of Leydig are unicellular glands of 

uncertain function in the epidermis of some larval urodeles. 

The color of the skin is often brilliant in many salamanders and frogs however certain 

tree frogs have a protective green coloration. The beautiful and strongly contrasted 

hues of the Spotted Salamander and of certain frogs are examples of warning colors; 

their conspicuous colors serve to warn off the predators that would otherwise devour 

them. The sudden flash and sudden disappearance of color in Phyllomedusa may be 

meant for confusing the possible predators. Apart from camouflage and warning, 

some colors have a sexual significance. The ground color of the skin of frogs can 

change as a result of environmental stimuli. The skin contains the deeply situated 

black melanophores, layers of guanophores, and yellow lipophores, which lie close 

below the epidermis. The various colors of the frog skin are produced both by 

pigments and purely physical phenomena and are under a neuro-endocrine control. 

The skin of modern amphibians lacks scales except in a few toads and in some of the 

burrowing, limbless caecilians. The skin is usually smooth and moist. Many 

burrowing Apoda have an exoskeleton made of small dermal scales. Similarly, bony 

exoskeletal plates occur in some Anura, beneath the skin of the back. In the anuran 

Xenopus and also in the urodele Onychodactylus, small, horny claws are found on the 

digits. The frog Leptodactylus pentadactylus, has horny chest grapples. However, 

apart from these few examples, the amphibian skin generally lacks hard parts. 

c. Alimentary canal- The wide mouth of anurans leads into a capacious buccal cavity, 

having internal nares in its roof, the eye bulges and the openings of the Eustachian 

tubes. The lips are little developed but the tongue is a characteristic organ for catching 

food and is a special feature required for terrestrial life. In Rana, it is attached to the 

floor of the mouth anteriorly and flicked outwards by its muscles. To keep it moist 

and sticky, a special intermaxillary gland is present. The saliva contains a weak 

amylase, which may serve to release sufficient substances for tasting. Special tracts of 

cilia carry the secretion from the intermaxillary glands to the vomeronasal organ and 

to the palatal taste buds. Behind the tongue is a slit like glottis. The teeth are small, 

fused to the bones and singly or doubly pointed. The teeth are without pulp or nervetissue. 

There are teeth only on the upper jaw of frogs, on the premaxillae, maxillae 

and vomers. They are used basically to prevent the escape of prey. 

Behind the buccal cavity is the pharynx, which leads by a short oesophagus or gullet into 

a stomach. Buccal cavity and oesophagus of anurans have mucus-producing goblet cells. 

The stomach consists of a wide cardiac and a narrow pyloric division (Figure 5). Its 

epithelium consists of mucus secreting cells and is folded, with simple tubular glands 

opening at the base of these folds. These glands, unlike those of mammals, are composed 

of only a single type of cell, which secretes both the acid and the pepsin found in the 

stomach. A pyloric sphincter guards the entrance into the duodenum, which is richly 

supplied with goblet cells. Here, the digested food is absorbed into the hepatic portal 

system. The remaining ileum is coiled and dilates into a large intestine or rectum that is 

24

special development for water absorption by terrestrial vertebrates. The rectum passes 

into the cloaca, the latter terminating into a cloacal aperture. 

The liver has two large lateral lobes and one small median lobe. Between its right and left 

lobes, occurs the gall bladder. Bile passes from the liver into the gall bladder via cystic 

ducts. Liver along with the fat bodies plays an important role in fat absorption. Pancreas, 

having both exocrine and endocrine functions, is held by mesenteries between stomach 

and duodenum. Digestive juices from the acinar cells flow down the pancreatic duct, 

which is bound to the bile duct and together they empty into the duodenum through a 

common opening. Paired thyroids are present below the floor of the mouth in front of the 

glottis and lateral to the hyoid apparatus. Removal of thyroids in tadpoles prevents 

metamorphosis. Furthermore, the periodic ecdysis / molting of the keratinous epidermal 

layers of the skin is under the control of anterior pituitary and thyroid, so that removal of 

either causes the cornified layers to remain unshed as a dark thick covering. On the other 

hand, administration of thyroid extracts will cause retardation of growth and bring about 

sudden metamorphosis in various anurans. Parathyroids occur as paired ovoid bodies, 

associated with calcium metabolism. The paired thymus gland situated behind and below 

the tympanic membrane is of doubtful function. On the ventral face of each kidney there 

occurs the elongated, yellow, compound adrenal / suprarenal gland. Spleen is a 

haematopoetic organ in anura, found attached to the anterior end of the rectum via a 

mesentery. 

Modifications in the alimentary canal of various amphibian groups are as follows: Few 

amphibians bite, though biting teeth are found in the adult Ceratophrys ornata. The South 

American tree frog Amphignathodon has teeth on lower and upper jaws and presumably 

has redeveloped them, a remarkable case of reversal of evolution. In many anurans, such 

as common toad, teeth are entirely absent. Most urodeles have teeth both on the upper and 

lower jaws. A deciduous fetal dentition occurs in ovoviviparous caecilians. These teeth 

are replaced by adult teeth at the time of parturition. Tongue in most urodeles is fixed and 

immovable while in several anurans it is free behind and attached in front. However, in 

Xenopus and Pipa, tongue is absent. The adhesive power of tongue in many frogs and 

some urodeles is enhanced by secretions of the lingual gland and the intermaxillary gland 

between the premaxillae and nasal capsule. Anurans are additionally equipped with a 

pharyngeal gland that discharges into the internal nares. The oesophagus may be ciliated 

and in the anurans, both buccal cavity and oesophagus possess mucus-producing goblet 

cells. Stomach glands of anurans secrete both pepsin and acid. The stomach is 

25

enormously distensible. The small intestine is a tube of almost uniform width in all 

amphibians. It is almost straight in Apoda, and is extremely long in certain tadpoles. A 

spiral valve does not occur, an increased absorption surface is obtained by great increase 

in length. There is some evidence that the liver is of unusual importance in fat storage. 

Certainly there is much evidence that amphibians can live for long periods without food. 

Axolotls, for instance, have remained alive for 650 days under conditions of starvation, 

losing about 80% of their former body weight. 

d. Respiratory organs and voice apparatus- A majority of amphibians have external 

gills in the larval state but these disappear in the adults of terrestrial forms. The 

perennibranchiate urodeles retain the external gills throughout life. These are 

branched cutaneous structures, richly supplied with blood vessels but not homologous 

to pharyngeal gills. On the other hand, internal gills are developed only in the larvae 

of Anura. They appear along the branchial arches below the external gills. 

In frog, the respiratory tract begins from the external nares, which are a pair of small 

apertures located on the snout. These lead into small nasal chambers lying in the skull. 

Vomeronasal / Jacobson’s organ is a pair of blind diverticula extending from the ventromedial 

part of the nasal chambers and acts as the accessory olfactory organ. The nasal 

chambers open into a buccopharyngeal cavity via small apertures called the internal 

nares / choanae. The buccopharyngeal cavity communicates with the small median 

laryngo-tracheal chamber with the help of a small slit like aperture known as the glottis. 

The laryngo-tracheal chamber leads into two very short tubes, the bronchi that terminate 

into the lungs. The latter are paired sac-like structures located in the anterior part of the 

body cavity on the sides of the heart. Lungs are pinkish in color, with thin, highly 

vascular, elastic walls. The inner surface possesses a network of low ridges called the 

septa. These enclose shallow depressions called the alveoli. Together, the septa and 

alveoli increase the respiratory area of the lungs (Figure 6 a). Histologically, the lung 

wall consists of the outermost layer called the peritoneum; the middle connective tissue 

layer and the inner epithelial layer. 

26

Respiratory movements involve the alternate raising and lowering of the floor of the 

buccal cavity, and are brought about by two sets of muscles, the sternohyal and the 

petrohyal muscles. Contraction of the sternohyal muscles causes a lowering of the 

buccal cavity. As a result, the buccopharyngeal cavity gets enlarged and air pressure 

inside it is reduced. Consequently, the outside high-pressured air immediately rushes 

into the buccopharyngeal cavity through the external nares, nasal chambers and 

internal nares. Thereafter, the premaxillae are raised due to the up-pushing of the 

lower jaw so that the external nares are closed. The petrohyal muscles now contract 

raising the floor of the buccal cavity. The reduced volume of the buccopharyngeal 

cavity forces the air into the lungs through the laryngo-tracheal chamber and the 

bronchi. In the lungs, exchange of gases occurs between the air in the alveoli and the 

blood flowing in the peripheral blood vessels. Air in the alveoli thus gets depleted of 

oxygen, becoming loaded with excess carbon dioxide. During expiration, floor of the 

buccopharyngeal cavity is lowered, the lungs contract and the glottis permits air from 

the lungs to enter the buccopharyngeal cavity. Finally the external nares open, 

followed by the raising of the floor of the buccopharyngeal cavity. The high-pressured 

air from within is expelled to the outside. Mouth and oesophagus are kept closed 

during pulmonary respiration. 

Modifications in the structure of lungs among the various amphibian groups are as 

follows: In frogs and toads, trachea is non-existent, an exception being the members 

of the family Pipidae where a definite trachea is present and the lungs function as 

hydrostatic organs. In urodeles, the trachea is usually short, though in Amphiuma and 

Siren, it is 4-5 cm long. In all amphibians, the tracheal cartilages are small and 

irregular in distribution. Trachea, whenever present, divides into two bronchi that lead 

directly into the lungs. Amphibians generally have simple lungs. In urodeles, the left 

lung is often larger than the right one. Alveoli may or may not be present or they may 

be restricted to the basal part of the lungs. However, in some salamanders, lungs are 

completely lacking and respiration is cutaneous and pharyngeal. In those species 

where it is present, the salamander lung also acts as a hydrostatic organ. In the 

perennibranchiate urodeles like Necturus, lungs as well as gills are simultaneously 

found in the adult. In Apoda, the left lung is very short, whereas, the right lung has 

alveoli all over the surface. The left lung is rudimentary in some Apoda, which 

instead have a tracheal lung. A larger alveolar respiratory surface is found in those 

amphibians, which are adapted to a terrestrial mode of life. 

Pulmonary respiration is used by a frog only during times of great oxygen need. 

Normally the oxygen requirement is met satisfactorily by cutaneous and 

buccopharyngeal respiration. It is believed that respiration through the skin accounts 

for almost 70% expulsion of carbon dioxide from the body. The characteristic feature 

of the moist amphibian skin is that it is highly vascular so as to function as an 

effective respiratory organ. Many amphibians show supplementary adaptations to 

cutaneous respiration. For instance, there is presence of highly vascular folds in 

Cryptobranchus. Similarly, the African Hairy frog, Astylosternus has extensive tracts 

of vascular papillae associated with the hind limbs of the male during the breeding 

season. These compensate for its reduced lungs and the greater need for oxygen 

during the reproductive period. The aquatic Typhlonectes has a richly vascular skin 

through which it respires. 

Amphibians are the first vertebrates that show evolution of a true voice apparatus. In 

frogs, larynx is represented by the laryngo-tracheal chamber (Figure 6 b) and is 

located near the posterior corner of the hyoid apparatus. At one end it communicates 

28

with the buccopharyngeal cavity through the glottis, while at the other end, it leads to 

the lungs by a short pair of bronchi. A framework provided by a pair of arytenoids 

and a single cricoid cartilage supports the glottis and walls of the laryngo-tracheal 

chamber. The cricoid cartilage is oval in shape while the arytenoids are semi lunar 

and lie inside the ring formed by the ovoid cricoid cartilage. The mucus membrane of 

the laryngeal chambers is raised into a pair of horizontal folds known as the vocal 

cords. These occur in both sexes, but are much better developed in the male anurans. 

A narrow space exists between the inner free edges of the vocal cords. It is through 

this space that the air has to pass on its way to lungs and back. The vocal cords vibrate 

as air is forced back and forth between lungs and the voice box resulting in the 

production of sound. The pitch of the sound is controlled by the level of the tension 

generated in the vocal cords. Vocal sacs are additional structures found in male frogs 

only. These are buccal diverticula connected with the mouth by small slit-like 

apertures and extend ventrally and laterally so as to lie under the outer skin and 

muscles of the throat region. The vocal sacs when fully dilated act as resonators. Male 

frogs are capable of producing sound under water as well as outside water. Voice is 

produced with the mouth closed. 

Among amphibians, the simplest type of larynx is found in certain urodeles like 

Necturus where only a pair of lateral cartilages encircles the glottis. Urodeles merely 

produce a hissing or squeaking sound. Other amphibians including frog show 

modification of these lateral cartilages by acquiring an anterior pair of arytenoids and 

the posterior cricoid cartilage. 

e. Blood vascular system- Double circulation, first introduced in lung fishes, also 

becomes a characteristic feature of amphibians (Figure 7). The auricle becomes 

completely divided into the left and right chambers by an interauricular septum. As a 

result, the two blood streams are kept separate in the auricles. The system of vessels 

draining the lungs to the heart and those going from the heart to the lungs is referred 

to as the pulmonary circulation. Other system of vessels through which blood is 

circulated to different body parts is known as the systemic circulation. Such a double 

circulation helps to cope with the terrestrial mode of life since the gills have been 

replaced by the lungs as respiratory organs. Secondly, the sinus venosus has shifted its 

position and opens into the right auricle. The third advancement in the amphibian 

heart is that the ventricle lining is thrown into many pocket-like structures formed by 

muscular bands. The incorporation of a complicated system of valves inside the 

auriculo-ventricular aperture is yet another improvement in the amphibian heart over 

that of fishes. All these 

features help to separate the 

aerated and the non aerated 

blood streams. 

The heart of frog is a muscular, 

reddish-colored conical organ that lies 

in the pericardial cavity ventral to the 

oesophagus and in front of a septum 

tranversum that completely separates 

the pericardial and coelomic cavities. 

The heart is ensheathed by the 

pericardium, a double-walled sac. The 

inner wall (epicardium) is applied to 

the heart surface. The two walls unite at the base of the arterial arches but around the 

29

heart they are separated by a pericardial space containing serous fluid. The pericardial 

fluid not only protects the heart from mechanical shock, but also allows easy 

contraction and relaxation movements. The anuran heart consists of a sinus venosus, 

right and left auricles, a single ventricle, and a conus arteriosus (Figure 8). The sinus 

venosus is a triangular, thin-walled chamber located in the dorsal heart surface with 

its apex directed backwards. It receives impure blood via the right and left pre-cavals 

and the post-caval veins. Blood is then delivered into the right auricle via the aperture 

guarded by two lip-like sinu-auricular valves. These valves guide the blood from the 

sinus venosus to the right auricle and prevent its backflow. The left auricle receives 

pure blood brought from the lungs via the pulmonary veins through a small aperture 

present in the left wall of the left auricle towards the anterior side. No valves however 

guard this aperture. Backflow of blood from the left auricle to the pulmonary veins is 

prevented by the oblique orientation of the pulmonary vein inside the wall of the 

auricle, which gets closed down whenever the wall of the auricle contracts. The two 

auricles are separated by an inter-auricular septum. 

The auricles contract 

and push their blood 

into the single ventricle 

via an aperture called 

the auriculo-ventricular 

aperture. It is guarded 

by four flap-like 

auriculo-ventricular 

valves (also known as 

the atrio-ventricular 

valves). Of the two 

larger valves, one arises 

from the dorsal border 

and the other from the 

ventral border of the 

auriculo-ventricular 

aperture. Fine thread-like structures called the chordae tendinae connect the free edges 

of the valves to the inner surface of the ventricle. The two smaller valves arise from 

the lateral walls of the auriculo-ventricular aperture. No chordae tendinae are attached 

to these. All these valves can open only towards the ventricle. The ventricle is a 

triangular, thick-walled, muscular structure. Its walls are raised up into muscular 

ridges or columnar carneae with interstices between them to prevent the mixing of 

blood of the two types of blood streams. Blood is thereafter driven into the conus and 

is expelled into the systemic arches. 

The conus arteriosus is a tubular chamber oriented obliquely on the ventral side of the 

right auricle. It originates from the right anterior border of the ventricle from the 

ventral side. At its base, the conus has three pocket-like semi lunar valves. These 

valves are arranged in a ring-like manner with their openings facing the conus. Blood 

can flow freely from the ventricle into the conus arteriosus. The main cavity of conus 

is divided internally into two unequal parts by a second ring of semi lunar valves. 

These parts include the long proximal pylangium and the short distal synangium. 

There is present a well-developed longitudinal spiral valve inside the cavity of the 

pylangium. Thus the internal cavity of the pylangium is incompletely divided into two 

30

parts: the cavum pulmocutaneum and the cavum aorticum. One part carrying pure 

blood leads into the carotid and systemic arches; the other carrying impure blood 

drains into the pulmo-cutaneous arches. The synangium is completely divided into 

two chambers: a dorsal chamber and a ventral chamber. The dorsal chamber sets up 

connection in front with the pulmo-cutaneous arches and behind with the cavum 

pulmocutaneum via an aperture located just in front of the spiral valve. The ventral 

chamber of the synangium also establishes connections in front with the carotid and 

systemic arches and behind with the cavum aorticum. 

Two sets of valves are present to 

prevent the backflow of blood: 

one at the junction of the conus 

and ventricle, the other between 

the conus arteriosus and the 

ventral aorta. Each ventral aortae 

is divided into three vessels: the 

anterior, carotid trunk; the 

middle, systemic / aortic trunk; 

and the posterior, pulmocutaneous 

trunk. The carotid 

divides into an external and 

internal branch, which supply 

the head. The systemic trunks 

curve around the oesophagus, 

the right arch becomes the dorsal 

aorta, the left continues as the 

coeliaco-mesenteric artery. The pulmo-cutaneous trunk divides into the pulmonary 

artery going to the lungs and the cutaneous artery going to the skin. In the tadpole, 

there are four pairs of aortic arches, associated with the gill capillaries. In the adult, 

the gills disappear. The 3 rd aortic arch loses its connection with the dorsal aorta and 

becomes the carotid trunk. The 4 th retains its connection with the dorsal aorta and 

becomes the systemic trunk. The 5 th disappears. The 6 th becomes the pulmo-cutaneous 

trunk (Figures 9 a, b). 

31 

The deoxygenated blood is 

returned from the head via the 

internal and external jugular 

veins that drain into the precaval 

vein. The latter also 

receives the brachial vein from 

the forelimb, and the musculocutaneous 

veins from the skin 

and muscles. Two portal systems 

occur. The blood from the hind 

leg is brought back by the 

femoral, that divides and its 

dorsal branch called as the renal 

portal receives blood from the 

sciatic vein and passes into the 

kidneys, breaking up into

capillaries. The ventral branch called as the pelvic vein joins with its opposite partner 

into the anterior abdominal vein, which divides into capillaries in the liver, where it is 

joined by the hepatic portal vein, bringing the blood from the stomach, intestine, 

spleen and pancreas. The renal veins that unite collect blood from the kidneys into the 

large post-caval vein. This passes through the liver, receiving the hepatic veins and 

finally opening into the sinus venosus (Figure 10). 

In the perennibranchiate urodeles, circulation is like that of a fish. The bulbus / ventral 

aorta gives off four afferent branchial arteries: 3 to the external gills and the 4 th that 

curves around the oesophagus and joins the dorsal aorta. From each gill arises the 

efferent branchial artery, all of which then unite into the dorsal aorta. Each afferent 

with the corresponding efferent artery constitutes an aortic arch. Carotids arise from 

the 1 st efferent artery and when the lungs arise, a pulmonary artery is given off from 

the 4 th aortic arch. When the gills atrophy, the 3 rd aortic arch becomes the carotid, the 

4 th becomes the systemic, the 5 th undergoes variable degrees of reduction, and the 6 th 

becomes the pulmonary artery. 

The urodeles show a transition from the fish-type to the frog type of venous system. 

The caudal vein brings the blood from the tail and it divides into the two renal portals 

that enter the kidneys; from the kidney blood drains into the paired cardinals. The 

anterior portions of the cardinals degenerate into two small azygous veins, receiving 

the blood from the back. Their posterior portions unite into the post-cava vein. The 

latter along with the hepatics, drains into the sinus venosus. Likewise, the iliac from 

the hind limb, divides into two branches: one joins the renal portal while the other 

forms the anterior abdominal and joins the hepatic portal. 

Amphibian RBCs are oval and nucleated. These arise either from the kidney or from 

the spleen or may arise from the bone marrow. White cells consist of large phagocytic 

macrophages, monocytes, phagocytic polymorpho-nuclear granulocytes and 

lymphocytes. 

In the closed system of vessels, as of vertebrates, the capillaries are separated from 

other body tissues by fluid-filled tissue spaces. These are filled with tissue fluid, 

which is essentially blood plasma that has seeped out of the capillaries. The tissue 

spaces communicate with minute lymph vessels that continue into larger vessels to 

32

make up the highly developed lymphatic system. Frogs are remarkable for the 

dilatation of many of its lymph vessels into large lymph sinuses. Between the skin and 

muscle are various spacious subcutaneous sinuses, separated from each other by 

fibrous partitions. The sinuses allow the frog’s skin to slide back and forth across the 

underlying structures. The dorsal aorta is surrounded by a large sub vertebral sinus. 

Lymph is driven through the system of lymph vessels into the venous system by 

means of lymph-hearts. The anterior pair is located beneath the supra-scapulae. The 

posterior pair is near the urostyle. The lymph hearts open into veins so that the lymph 

fluid can be again mixed with the general circulation. 

f. Endoskeleton- The axial skeleton of frog includes the skull, vertebral column, ribs 

and sternum while the appendicular skeleton consists of the pectoral and pelvic 

girdles along with bones 

of the forelimbs and the 

hind limbs (Figure 11). 

The skulls of many early 

tetrapods retained certain 

features of their piscine ancestors 

but those of modern amphibians 

show considerable deviation. 

There has been a reduction in the 

number of bones as well as a 

general flattening of the skull. 

The auditory capsule bears a 

ventral opening, the fenestra 

ovalis, into which fits the bony / cartilaginous stapedial plate of the columella, derived 

from the hyomandibular cartilage. The columella has developed in connection with 

the evolution of the sense of hearing and with the change from the hyostylic to the 

autostylic method of jaw suspension in which the hyomandibular loses its significance 

as a suspensorium. The chondrocranium persists to a considerable extent in 

amphibians, but some of it has been replaced by cartilage bones. Basioccipital and 

supraoccipital regions are not ossified. The atlas is articulated with the skull by a pair 

of occipital condyles, projections of the exoccipitals. Basisphenoid and presphenoids 

also are not ossified. Prootics, and sometimes the opisthotics are ossified and fused to 

the exoccipitals. Membrane bones form the greater part of the roof of the skull. They 

are no longer closely related to the integument and occupy a deeper position in the 

head than in the fishes. A large membrane bone, the parasphenoid, covers much of the 

ventral part of the chondrocranium. The quadrate in amphibians is fused to the 

auditory region of the skull. Palatine and pterygoid membrane bones, which form 

about the anterior part of the palatoquadrate, are well developed. The outer arch of 

membrane bones is represented by pre-maxillaries and maxillaries. Anurans also have 

a quadratojugal. The lower jaw consists of a core of Meckel’s cartilage surrounded by 

membrane bones. The remaining part of the visceral skeleton is reduced in 

comparison with fishes. The main features of a frog’s skull are as follows: 

33

Skull of frog (Figure 12) 

1. Skull is triangular, dorso-ventrally flattened and broad. 

2. It is dicondylic, with two occipital condyles that articulate with the atlas vertebra. 

3. Occipital region is greatly reduced. 

4. The cranium is small and narrow. 

5. The skull is platybasic lacking an inter-orbital septum and the cranium extends 

beyond orbits. 

6. The fronto-parietal is present on the roof of the cranium. 

7. The nasals are large triangular bones covering the olfactory capsules. 

8. The sphenethmoid extends forward into the region of olfactory capsule and is partly 

covered by the fronto-parietal and nasals above and the parasphenoid below. 

9. Parasphenoid is a dagger-shaped bone forming the floor of the cranium. 

10. Vomers lie beneath the nasals and bear vomerine teeth. 

11. Upper jaw consists of premaxillae, maxillae and quadratojugals. 

12. Lower jaw consists of dentaries and angulosplenials. 

13. The suspensorium is autostylic, with the lower jaw attached to the skull through rodlike 

quadrate cartilage. 

14. Basisphenoids, alisphenoids, presphenoids and supra-and basioccipitals are absent. 

15. Prootic bones are present on the sides of the exoccipitals. 

16. Squamosals are T-shaped bones present on the dorsal side. 

17. Pterygoids lie opposite to the squamosals on the ventral side. 

18. Palatines are rod-shaped bones present on the ventral side with one end touching the 

maxilla and the other in contact with the sphenethmoid. 

34

Bones seen on the dorsal surface: 

Premaxillae, maxillae, quadratojugals, squamosals, septomaxillaries, nasals, fronto-parietals, 

prootics, exoccipitals and occipital condyles 

Bones seen on the ventral surface: 

Premaxillae, maxillae, quadratojugals, vomers, palatines, sphenethmoids, parasphenoid, 

pterygoids and exoccipitals 

The skull of urodela differs from that of the frog in many ways: The trabeculae do not meet 

either below the brain to form a basis cranii or above it to form a cranial roof. There is, 

above, a huge superior cranial fontanelle, and below an equally large basicranial fontanelle. 

The former is covered, in the complete skull, by the parietals and frontals, the latter by the 

parasphenoid. The parietals and frontals are separate. The parasphenoid is not T-shaped. A 

single bone, the vomeropalatine bearing teeth, represents the palatine and vomer. The hyoid 

arch is large and its dorsal end may be separated as a hyomandibular. There are three or four 

branchial arches. The stapes has no extra-columella and no tympanic cavity or membrane. In 

some anuran species, there is the presence of small supra and basi-occipitals. In others, 

investing bones of the roof are very strongly developed. In the apoda, the investing bones are 

very large and form a substantial structure. 

The vertebral column is divisible into: the cervical region, an abdominal / thoracolumbar 

region, a sacral region and the caudal region. The total number of vertebrae in urodeles and 

apoda may be as much as 250, while there are only 9 vertebrae and a single rod shaped 

caudal bone called the urostyle in anurans. In the lower urodela the centra are biconcave as in 

fishes. However, the neural arches are much better developed than in any fish and have well 

developed zygapophyses. The apoda also have biconcave vertebrae but in the higher 

urodeles, the anterior surface of the centrum has a convexity while the posterior surface 

retains its concavity, thus forming a ball and socket joint and the condition is known as 

opisthocoelous. In the anura the condition gets reversed with respect to the anterior and 

posterior surfaces and this condition is called as procoelous. A frog’s vertebral column 

includes the following bones: 

Vertebrae of frog (Figure 13) 

35 

Atlas vertebra 

1. The first vertebra is called the atlas. 

2. It is small and ring-like in form. 

3. Centrum and neural spine are 

reduced. 

4. Transverse processes and 

prezygapophysis are absent. 

5. The neural arch is large. 

6. The anterior face of centrum 

possesses a pair of concave facets for 

the articulation with the occipital 

condyles of the skull. 

7. The posterior margin of the neural 

arch bears a pair of 

postzygapophyses.

Typical second vertebra 

1. In frog the 2 nd vertebra is typical in structure. 

2. The centrum is procoelous (concave on the anterior face and convex on the 

posterior face). 

3. It is ring-like having a large hole called the neural canal. 

4. The solid arch on the dorsal side of the ring is called the neural arch. 

5. The neural arch bears a small, mid-dorsal neural spine, which is directed 

backwards. 

6. Transverse processes are broad and wing-like. 

7. A pair of small upwardly and inwardly directed articular facets called the 

prezygapophyses is present on the anterior margin of the neural arch. 

8. A pair of small downwardly and outwardly directed postzygapophyses is present 

on the posterior margin of the neural arch. 

Third and Fourth vertebrae 

These vertebrae resemble the typical vertebra in structure except slight variations. 

1. The transverse processes are stout and elongated. 

Fifth, Sixth and Seventh vertebrae 

These vertebrae also resemble the typical vertebra in structure except slight difference. 

1. The transverse processes are pointed. 

Eighth vertebra 

Ninth vertebra 

1. The centrum is amphicoelous (biconcave on both the sides). 

2. The anterior concavity receives the posterior convexity of seventh vertebra. 

3. The posterior concavity receives the anterior convexity of ninth vertebra. 

4. Transverse processes are pointed and upwardly directed. 

5. Prezygapophyses and postzygapophyses are present on the anterior and 

posterior margins respectively. 

1. Ninth vertebra is also known as sacral vertebra. 

2. The centrum is biconvex, i.e., convex on both the sides (bearing one convexity 

anteriorly and two convexities posteriorly). 

3. The anterior convexity fits into the posterior concavity of the eighth vertebra. 

4. The posterior convexities fit into the anterior concavities of urostyle. 

5. Transverse processes are cylindrical, stout and backwardly directed. 

6. An iliac facet is present at the tip of each transverse process for the articulation of 

ilium bone of pelvic girdle. 

7. Neural spine is inconspicuous, i.e., greatly reduced. 

8. Prezygapophyses are well developed along the anterior end of neural arch, while 

the postzygapophyses are entirely absent. 

36

Urostyle 

1. Urostyle is the Xth vertebra representing the caudal region in the frog. 

2. It is long and triangular with a pointed apex directed backwards. 

3. Centrum is long, rod-like with a broad anterior end bearing two concavities to 

receive the convexities of IXth vertebra. 

4. Dorsally it is raised into a vertical ridge gradually tapering posteriorly. 

5. Anteriorly the vertical ridge contains a short, narrow canal for spinal cord. 

6. Transverse processes, pre-and postzygapophyses are entirely absent. 

The pectoral girdle has basically the same structure, as found in the other pentadactyl 

craniates. The scapula is ossified, and is connected by its dorsal edge with a suprascapula, 

formed partly of bone and partly of cartilage. The coracoid is also ossified, but the 

precoracoid is cartilaginous and has an investing bone called clavicle associated with it. The 

cartilaginous epicoracoid connects the coracoid and precoracoid ventrally. The rounded head 

of the humerus fits into the glenoid cavity. Passing forwards from the anterior ends of the 

united epicoracoids, is a bony structure, the omosternum, which expands into a cartilage plate 

called the episternum; and passing backwards from the epicoracoids is another bony rod, the 

mesosternum, which is also associated with a cartilage structure known as the xiphisternum. 

This is the first indication of a sternum in the terrestrial vertebrates; however, the sternal 

apparatus of the amphibians differs developmentally from the sternum of higher vertebrates. 

Pectoral girdle and sternum of frog (Figure 14) 

5. The scapular portion comprises the supra-scapula and scapula. 

6. Supra-scapula is a thin cartilaginous plate on the dorsal side. 

1. Pectoral girdle is present in the 

thoracic region (shoulder region) 

and provides attachment to the 

fore-limbs and their muscles. 

2. It protects the inner softer parts of 

the thorax. 

3. It consists of two similar halves 

united mid-ventrally and separated 

dorsally. 

4. Each half is divided into a dorsal 

scapular portion and a ventral 

coracoid portion. 

7. Scapula is a bony plate having a glenoid cavity into which articulates the head of 

humerus. 

8. The coracoid portion comprises the clavicle, coracoid, precoracoid and epicoracoid. 

9. Clavicle and coracoid meet mid-ventrally with the sternum and their counterparts of other 

side by a strip of cartilage-the epicoracoid. 

37

10. The sternum lies in the mid-ventral line. It consists of episternum, omosternum, and 

xiphisternum. 

11. The episternum is a flat, almost circular plate of cartilage. 

12. The omosternum is a bony rod connected to the episternum on the anterior side and 

clavicle on the posterior side. 

13. The mesosternum is a cartilaginous rod lying opposite the omosternum. 

14. The xiphisternum is the terminal broad cartilaginous plate lying at the tip of the 

mesosternum. 

The forelimbs show the fusion of the radius and ulna into a single radius-ulna. There are only 

four complete digits with a vestigial one, the prepollex. Only six carpals are present. 

Forelimb bones of frog (Figure 15) 

Humerus 

1. It is the bone of fore-limb and is the 

component of upper-arm. 

2. It is a short, stout and cylindrical 

bone with a slightly curved shaft. 

3. Its proximal end is known as the 

head which fits into the glenoid 

cavity of pectoral girdle. 

4. The head is covered with calcified 

cartilage. 

5. The ridge below the head is known 

as deltoid ridge. 

6. The distal end forms a rounded trochlea with a condylar ridge on either side. 

7. The trochlea articulates with the groove of radius-ulna. 

Radius-ulna 

1. It is a compound bone of fore-limb and is the component of fore-arm. 

2. It is formed by the fusion of radius and ulna bones. 

3. Its proximal end has a concavity to receive the trochlea of humerus. 

4. The ulna projects into an olecranon process. 

5. The distal portion of radius-ulna is somewhat flat having a groove. 

6. Distal portion has an articular surface for the metacarpals. 

Carpus-metacarpus and digits 

1. The bones of the wrist are called carpals. 

2. The carpal bones are six in number and arranged in two rows of three each. 

3. The bones of the proximal rows are called ulnare, intermedium and radiale. These 

bones articulate with the radius-ulna. 

38

4. The bones of distal row are called capitato-hematum, trapezoid and trapezium. These 

bones articulate with the metacarpals. 

5. The hand is provided with five slender metacarpals. The first metacarpal is 

rudimentary. 

6. The digit corresponding to the thumb is absent. 

7. The remaining four metacarpals are supported by phalanges. 

8. The second digit bears 2 phalanges. 

9. The third and fourth digits bear 3 phalanges each. 

The pelvic girdle is peculiarly modified. The girdle has two long, curved bars articulating in 

front with the transverse processes of the sacral vertebra and uniting posteriorly in an 

irregular vertical disc of mingled bone and cartilage. The disc bears on each side a deep 

acetabulum into which fits the head of the femur. The ilia together form a long jointed lever 

especially adapted for jumping. The frog lacks a stabilizing tail, and its center of gravity is 

located just behind the sacrum. 

Pelvic girdle of frog (Figure 16) 

1. Pelvic girdle lies in the posterior 

region of the trunk. 

2. It gives support to the hind-limbs. 

3. It is V-shaped and composed of two 

similar halves each of which is 

known as an os-innominatum. 

4. Each os-innominatum is composed 

of three bones called the ilium, pubis 

and ischium. 

5. Ilium is greatly elongated and forms the major part of each os-innominatum. It runs 

forwards to meet the transverse processes of the ninth vertebra. 

6. It bears a prominent vertical 

ridge called the iliac crest on its dorsal surface. 

7. Pubis is much reduced. It is a triangular piece of calcified cartilage. 

8. Ischium is a larger and slightly oval bone. 

9. The disc formed by the union of the three 

acetabulum. 

10. The head of the 

femur fits into the acetabulum. 

bones contains a cup-shaped cavity called the 

In the hind limb, the tibia and fibula are fused to form a single tibia-fibula. The two bones in 

the proximal row of the tarsus, namely the tibiale or astragalus and the fibulare or calcaneum, 

are greatly elongated and provide the leg with an extra segment. There are three tarsals in the 

distal row. There are five well-developed digits. On the tibial side of the first there is an 

additional spur-like structure or calcar. This extra digit is known as the prehallux. 

39

Hind-limbs bones of frog (Figure 17) 

Femur 

1. Femur 

is the bone of thigh region of 

hind-limb. 

2. It is long and slender having a slightly 

curved shaft. 

3. The proximal swollen end is called 

the head. 

4. Head fits into the acetabulum of 

pelvic girdle. 

5. The distal end forms a condyle which 

articulates with the tibia-fibula. 

6. The head and condyle are covered by 

calcified cartilage. 

Tibia-fibula 

1. Tibia-fibula 

is a compound bone of 

the shank region of hind limb. 

2. It is formed by the fusion of tibia and 

fibula bones forming a single bone 

called the tibia-fibula. 

3. The proximal and distal ends are covered by cartilage. 

4. Near the proximal end tibia bears a cnemial or tibial crest. 

5. The proximal end articulates with the astragalus-calcaneum. 

Astragalus-calcaneum 

1. Astragalus-calcaneum 

is a compound bone of ankle of hind-limb. 

2. The ankle consists of two rows of four bones. The first or proximal 

row consists of 

two long bones fused together at their proximal and distal ends with a wide gap in the 

middle. 

3. The inner bone is thinner and slightly curved called the astragalus or tibiale. 

4. The outer bone is thicker and straight called the calcaneum or fibulare. 

5. The proximal and distal ends are covered by epiphyses of calcified cartilage. 

Metatarsals 

and digits 

1. The foot of frog is supported by five metatarsals bearing five true toes. 

2. The metatarsals are long and slender bones. 

3. The first, second, third and fourth metatarsals bear 3 phalanges each. 

4. A small preaxial sixth toe composed of 2 or 3 bones is present on the inner 

side of the 

first toe. 

5. The sixth toe is called the prehallux. 

Mo difications of the appendicular skeleton in different amphibians are as follows: The 

shoulder girdle of urodela has unossified coracoids of great size. The precoracoid is also 

large, and there is no clavicle. The sternum is a rhomboid cartilage plate and there is no 

omosternum. The pectoral girdle of anurans shows several modifications. In many frogs the 

40

two halves are firmly united in the mid-line and are closely related to the sternum (the 

firmisternal condition). In toads and some frogs the two halves overlap in the middle (the 

arciferal condition). The overlapping of the coracoids is sometimes correlated with the 

absence of an omosternum as in urodela. In the pelvic girdle of the urodela the combined 

ischiatic and pelvic regions are united into elongated cartilaginous plates on each side, which 

give rise to the rod-like ilia. In many urodela and some anurans there occurs a cartilage rod 

called the epipubis, which is attached to the anterior border of the pubic region. The limbs of 

urodela differ from that of anurans. There are usually four digits in the forelimb and five in 

the hind limb. In Anura, the limbs are modified by the fusion of the radius and ulna and of the 

tibia and fibula, and by the great elongation of the two proximal tarsals. A prehallux is 

usually present. 

g. Nervous system and sensory organs- Brain: Frog’s brain includes three parts: fore, 

mid and hind brain (Figure 18). The fore brain consists of a pair of olfactory lobes, a 

pair of cerebral hemispheres and a diencephalon. The olfactory lobes located at the 

anterior most part of the brain are large-sized and fused at the median line. The 

olfactory fibers from the nasal sac pass into the cerebral hemispheres through the 

olfactory lobes. A pair of cerebral hemispheres is located behind the olfactory lobes. 

These large elongated structures are separated from each other by a median 

longitudinal groove. Floor and lateral walls of the two hemispheres are thickened to 

form the corpora striata. Two commissures, the anterior commissure and the 

hippocampal commissure join the hemispheres. Each cerebral hemisphere encloses a 

cavity called the lateral ventricle and the lateral ventricles communicate with the 

olfactory ventricles in front and the third ventricle below via the foramen of Monroe. 

The cerebral hemispheres receive tactile and optic impulses from the skin receptors 

and the eyes. The diencephalon or the thalamencephalon is placed behind the cerebral 

hemispheres and prior to the mid brain. It encloses the third ventricle and has a non 

nervous roof that forms a thin vascular covering called the anterior choroid plexus. 

Behind the choroid plexus is a small hollow outgrowth called the pineal stalk, which 

during the larval stage bears a pineal body. The floor of the third ventricle is called 

the hypothalamus. The optic chiasma is found ventral to the hypothalamus. A hollow 

median bilobed process called the infundibulum is found just behind the optic 

chiasma and bears a flat oval body called the hypophysis. The infundibulum together 

with the hypophysis constitutes the pituitary body. Diencephalon receives a number of 

afferent optic fibers from the 

eyes and some from the skin 

receptors. 

41 

The mid brain consists 

of a pair of 

large optic lobes and a thick crura 

cerebri. The optic lobes are a pair of 

oval bodies on the dorsal side of the 

brain and have a pair of cavities 

called the optic ventricles, which 

open into a narrow passage called 

the iter or aqueduct of Sylvius. The 

optic lobes receive fibers of optic, 

olfactory and auditory nerves. The 

crura cerebri are a pair of thick 

bands of nerve fibers extending 

antero-posteriorly below the optic 

lobes. These connect the

diencephalon with the medulla oblongata. 

The hind brain consists of the cerebellum and the medulla oblongata. Cerebellum is a 

narrow band-like structure on the dorsal side of the brain just behind the optic lobes and 

encloses the cerebellar ventricle. Cerebellum is less developed in frogs and not 

differentiated into lobes. Medulla oblongata is the posterior most part of the brain that is 

thick at its anterior end but tapers posteriorly into the spinal cord. It encloses the fourth 

ventricle that communicates with the iter in front and with the central canal of the spinal 

cord behind. 

The brain is enclosed in two meninges, the inner thin and highly vascular pia-arachnoid 

membrane and the outer thick and tough dura mater. Between the two meninges is the 

subdural space filled with cerebrospinal fluid. Between the dura mater and the bony wall 

of the cranial cavity lies another space called the epidural space, also filled with 

cerebrospinal fluid. There are ten pairs of cranial nerves in anamniotes; a detailed chart of 

the nerves is given below: 

Table 7: Cranial nerves of frog 

S. No. Roman Number Nomenclature Nature Origin / Supply 

1 I Olfactory Sensory Olfactory epithelium 

2 II Optic Sensory Eye 

3 III Oculomotor Motor Four of the six eye muscles 

4 IV Trochlear Motor Superior oblique eye 

muscle 

5 V Trigeminal Mixed Head and jaw muscles 

6 VI Abducens Motor Posterior rectus eye 

muscles 

7 VII Facial Mixed Muscles of the face 

8 VIII Auditory/ 

Acoustic 

Sensory Internal ear 

9 IX Glossopharyngeal Mixed Tongue and pharynx 

10 X Vagus Mixed Viscera 

Sensory Organs: 

The Olfactory organs show modifications correlated with the terrestrial life. Each olfactory 

chamber has an external nostril and an internal nostril, the latter opens into the mouth. These 

two openings are separated by a nasal septum. The olfactory passage in amphibians is short. 

In aquatic forms the passage is lined with folds. Olfactory sense cells are found in the 

depressions between these folds, and ciliated epithelium covers the ridges. The olfactory 

epithelium in terrestrial forms is located in the upper medial part of the nasal passages. It is 

42

not folded to any extent. In some forms, a shelf-like fold from the lateral wall foreshadows 

the appearance of the conchae or the turbinal folds, which become highly developed in more 

advanced vertebrates. Glandular areas in the nasal passages keep the olfactory epithelium 

moist. In addition, a new olfactory structure first appears in amphibians and is known as the 

Jacobson’s / vomeronasal organ, which communicates with the olfactory chamber and the 

buccal cavity. It arises as a ventro-medial or ventro-lateral evagination of the nasal passage 

and is believed to be used in testing food substances held in the mouth. It is supplied with 

branches of the terminal, olfactory and trigeminal cranial nerves. 

Gustatoreceptors: Taste buds of amphibians are found on the roof of the mouth, tongue and 

on the lining of the jaws. In frogs, they lie on the free surfaces of the fungiform papillae. The 

more numerous filiform papillae do not have taste buds associated with them. 

Lateral line sense organs occur in the larval stage and these are receptive to water vibrations. 

Branches of facial, glossopharyngeal and vagus nerves supply these. These sensory structures 

however disappear during metamorphosis. 

The Visual sensory organs: Among amphibians, the anuran eyes are best developed and are 

lodged in the orbital fossae on either side of the head. The upper eyelid is large and 

immobile, while the lower eyelid forms a protective nictitating membrane, which covers the 

eye whenever the animal is in water (Figure 19). The movement of the nictitating membrane 

is regulated by the retractor bulbi and the levator bulbi muscles. The eyeball consists of three 

concentric layers: the outermost fibrous tunic, middle vascular uvea and the innermost 

nervous retina. The large posterior part of the fibrous layer is called the sclerotic while the 

small anterior transparent part in front is the cornea. The curved surface of the cornea helps 

the lens in focusing the light rays. Uvea is made of loose connective tissue having blood 

capillaries and pigment cells. Its part lining the sclerotic is called the choroid. At the junction 

of the sclerotic and cornea is the ciliary body but ciliary muscles are absent. The uvea in front 

separates from the sclerotic to form the pupil, which is perforated in the middle by an 

aperture called the pupil and has sphincter and dilator muscles to regulate the amount of light 

entering in the eye by changing the size of the pupil. The iris divides the cavity of the eyeball 

into a small anterior aqueous chamber and a large posterior vitreous chamber. 

The innermost delicate part of the eye is called the retina. Its optic part consists of an outer 

pigmented layer adjacent to the choroid and the inner nervous layer having receptor cells and 

neurons. The inner sensory area has three regions: the outer light-sensitive cells, the middle 

bipolar nerve cells and the inner ganglion cells. Rods and cones are found in the light 

sensitive cells and contain the visual pigments, rhodopsin and iodopsin. An area called the 

macula lutea or yellow spot is the small part of retina just opposite the center of pupil and is 

the point of most acute vision. However, frog eye lacks the fovea centralis depression. The 

lens is enclosed in a delicate lens capsule and lies just behind the iris. 

The light rays entering the eyeball are focused on the retina by a combination of the 

conjunctiva, cornea, aqueous humor, lens and vitreous humor. The inverted image formed on 

the retina stimulates the receptor cells in the area centralis, which in turn generates nerve 

impulses that are conveyed to the brain by the optic nerve. The eyes of a frog have limited 

focusing power. The frog is short sighted on land but far sighted in water. 

Auditory sense organs: The ears of frog located behind and below the eyes are organs of 

hearing and equilibrium. Each ear consists of the middle and the inner ear. External ear is 

absent. The middle ear is visible from outside and encloses an air filled cavity called the 

tympanic membrane. It is limited internally by the auditory capsule and externally by the 

43

tympanic membrane or tympanum. The cavity of the middle ear communicates with the 

pharynx by a narrow passage called the Eustachian tube, which serves to conduct the external 

sound waves into the ear and also keeps the air pressure inside the tympanic membrane equal 

to that outside it. Externally, the cavity of the middle ear is limited by the tympanic 

membrane, which is vibratile in nature. A club-shaped columella auris touches the center of 

the tympanic membrane and extends across the tympanic cavity to a cartilaginous stapedial 

plate, which is fused with an aperture in the auditory capsule called the fenestra ovalis. A 

ring-like bone called the operculum is present in the fenestra ovalis and is covered by a 

membrane attached to the scapula by a muscle. 

44

The inner ear is enclosed inside a bony auditory capsule, the latter is filled with a watery fluid 

called the perilymph in which floats the soft and compact inner ear / membranous labyrinth. 

The latter consists of an irregular structure called the vestibule that is divided by a shallow 

constriction into an upper large chamber called the utriculus and a lower small chamber 

called the sacculus (Figure 19). Three small diverticula collectively known as the lagena are 

given out by the sacculus. The lagena represents the coiled cochlear duct of the mammals. 

The utriculus bears three semicircular canals, which are oriented at right angles to each other 

thus assuming mutually perpendicular planes. At least one end of each semicircular canal is 

always swollen into a small round structure called the ampulla. The hollow, membranous 

labyrinth is filled with a fluid called endolymph and contains pieces of calcium carbonate in 

the form of ear stones or otoliths. The wall of inner ear is lined by cubical epithelium that is 

modified to form sensory spots at certain places. Each ampulla as well as sacculus, utriculus 

and lagena possesses one sensory spot each. The sensory spots of the ampullae are called the 

cristae while those of sacculus, utriculus and lagena are called the maculae. The macula of 

utriculus is the large pars neglecta while the macula of the lagena is known as the basilar 

papilla. Cristae and maculae consist of sensory cells associated with supporting cells. Each 

sensory cell has a tapering hair-like process at its free end and a nerve fiber at its lower end. 

Sound waves first strike the tympanum setting it into vibrations. These vibrations are 

conveyed to the columella auris and the stapedial plate. The perilymph conducts the 

vibrations to the endolymph, which in turn brings about disturbance of the sensory hair of the 

maculae in the sacculus and lagena. The influenced auditory nerves convert these vibrations 

into nerve impulses that are transmitted to the brain. As far as equilibrium is concerned, the 

semicircular ducts along with utriculus help in maintaining the correct body posture. 

Whenever the head is tilted, it alters the stress transmitted by the otoliths to the sensory hair. 

This pressure change works as a stimulus for the auditory cells. The sensory cells in turn 

generate a nerve impulse, which is carried to the brain by the auditory nerve. The disturbance 

of equilibrium in any direction is detected by the cristae of the semi-circular canals that are 

arranged in three different planes. 

Modifications of the nervous system and the sensory organs among the various amphibian 

groups are as follows: The urodele brain is more elongated and slender, with small optic 

lobes and having a non-union of olfactory bulbs. The Jacobson’s organs are usually found; 

the olfactory sacs open into the mouth by external nostrils located behind the vomers. The 

eye has modifications for allowing long-sightedness in the terrestrial forms, while the aquatic 

forms still retain the lachrymal ducts. Eyelids are vital in the terrestrial forms, but are absent 

in a few primitive forms. Urodeles, Apoda and some Anura have no tympanic cavity and 

tympanic membrane; one or both the middle ear ossicles may also be absent. In some 

urodeles and anurans, the highly reduced stapes is associated with a cartilage inserted in the 

fenestra ovalis and called the operculum. Possibly, vibrations from the forelimbs are 

conveyed to the operculum and the passed on to the inner ear. Lateral line organs are retained 

in the perennibranchiate urodeles and in the larvae of the terrestrial forms. Groups of 

neuromast organs are found in the aquatic anurans that possibly correspond with those of 

fishes. 

h. Urinogenital system and Osmoregulation- The primitive archinephric type of 

kidney found in the larval stage of the hagfish (cyclostomes) also occurs in the larval 

caecilians in which there is a distinct metameric arrangement of kidney tubules, renal 

corpuscles, and nephrostomes. In the adult apoda, the opisthonephros extends the 

greater part of the length of the coelom and is lobulated. Although a small head 

45

kidney with peritoneal connections is present in many larval amphibians, it does not 

persist in the adult stage. 

Urodeles have opisthonephric kidneys much like those of elasmobranchs. The kidney 

consists of two regions: an anterior portion, which in males is concerned more with 

genital than urinary functions, and a posterior expanded urinary region, which makes 

up the main part of the opisthonephros. Numerous collecting ducts or tubules leave 

the opisthonephros at intervals to join the persisting archinephric duct. The latter, in 

both sexes open on either side of the cloaca at the apex of a small papilla. In Necturus 

peritoneal connections with some of the kidney tubules persist throughout life. 

The opisthonephric kidneys of anurans show a more posterior concentration of 

tubules and are confined to the posterior part of the abdominal cavity. The kidneys are 

flat, oval, dark-red organs, in the posterior region of the coelom. They are dorsally 

located, retroperitoneal, and flattened in a dorso-ventral direction. There is no clearcut 

distinction between the anterior and posterior ends, as in urodeles. An adrenal 

gland of a yellowish-orange color is located on the ventral side of the kidney. Blood 

comes to the kidney from two entirely different sources. The adult frogs have ciliated 

nephrostomes on the ventral surfaces of the kidneys. They are usually not connected 

with kidney tubules but have become secondarily connected with the renal veins. The 

kidneys of female anurans have no relation to the reproductive system, but in males 

an intimate connection exists. Certain anterior kidney tubules have become modified 

as efferent ductules connecting the testis with the kidney and archinephric duct, the 

latter serves to transport spermatozoa as well as urinary wastes. Unlike the condition 

in urodeles, the archinephric ducts are located within the kidney along its lateral 

margin. They leave the opisthonephros near the posterior near the posterior end and 

pass to the cloaca. The structure of a typical renal tubule of an amphibian kidney is 

shown in Figure 20. A thin-walled urinary bladder connects with the amphibian 

cloaca a short distance beyond the openings of the archinephric ducts. It is bilobed. 

There is no direct connection of the ducts with the bladder, so that the urine first 

passes into the cloaca. 

46

In spite of several terrestrial adaptations incorporated into the amphibian body, the 

kidneys of extant forms retain fish-like characters. However, when on land, the loss of 

water is retarded by the reduction of glomerular filtration by a hormone from the pars 

neuralis, which constricts glomerular arterioles. Water conservation is also brought 

about by the passive absorption of water and salts through the integument. Certain 

47

desert-dwelling frogs like Cyclorana, Notaden, and Heleioporus can survive severe 

droughts by aestivating in dry surface mud and can absorb water very rapidly. 

Furthermore, their kidneys have venous sinuses into which drain peritoneal funnels. 

During aestivation, water stored in the peritoneal cavity is drawn through these 

channels into the general circulation. It is also known that in Cyclorana the glomeruli 

are reduced in size and vascularity. 

Male reproductive tract: The shape of amphibian testes shows a correlation with the 

body shape. In caecilians, these are elongated structures, resembling a string of beads. 

Each swelling consists of masses of seminiferous ampullae, all of which are 

connected by a longitudinal collecting duct. In urodeles, the testes are shorter and 

irregular in outline. In anurans, these are compact, oval or rounded structures (Figure 

21 b). A marked difference in size is visible during the breeding and non breeding 

season. Fat bodies associated with the testes, show a fluctuating size correlated with 

the seasons. 

The relationship of the reproductive and excretory systems in male amphibians is 

closer than in most fishes. Efferent ductules usually join a longitudinal canal inside 

the testis or along its medial border. The efferent ductules move through the 

mesorchium, enter the anterior part of the opisthonephros on its medial side, and may 

connect directly to the archinephric duct or join certain kidney tubules, which in turn 

connect to the archinephric duct. In urodeles, efferent ductules join a narrow 

longitudinal Bidder’s canal, which moves within the mesorchium, but outside the 

medial edge of the kidney. Bidder’s canal connects by a number of short ducts to 

48

kidney tubules in the narrow anterior part of the opisthonephros. Certain kidney 

tubules emerge from the lateral edge of the opisthonephros and join the archinephric 

duct that moves posteriorly. The anterior part of the archinephric duct is concerned 

mainly with the transport of spermatozoa, but the posterior part serves for the 

elimination of urinary wastes also. The archinephric ducts enter the cloaca 

independently. 

Conditions in anurans are same as those found in urodeles, but with some variations. 

Efferent ductules enter the anterior end of the opisthonephros along its medial edge. 

In some forms they connect directly to the archinephric duct, but in others join the 

Bidder’s canal, which lies within the opisthonephros close to its anterior border. 

Spermatozoa are then transported from the Bidder’s canal through kidney tubules to 

the archinephric duct, which also courses within the opisthonephros but along its 

lateral border. (A Bidder’s canal of unknown function is also found in the female 

kidney). The archinephric duct emerges from the kidney near its posterior end and 

passes to the cloaca. In males of several species, a dilatation of the archinephric duct 

near the cloaca forms a seminal vesicle in which spermatozoa may be stored 

temporarily. Copulatory organs are absent in urodeles and anurans. In some 

caecilians, the muscular cloaca is protrusible and serves as a type of intromittent 

organ when the cloacae are in apposition. 

Female reproductive tract: Amphibian ovaries are saccular structures, with the shape 

varying with the body shape. They are long and narrow in caecilians; elongated in 

urodeles; shortened and more compact in anurans (Figure 21 a). The cavity within 

each ovary becomes lymphoid in character and the ova escape into the coelom 

through the external walls of the ovaries. Fat bodies are closely associated with 

amphibian ovaries, serving for the storage of nutriment. A peculiar structure in the 

male toad, known as the Bidder’s organ, may under certain conditions develop into a 

true ovary. Oviducts in amphibians have the same structural pattern throughout the 

class. These are paired elongated tubes with ostia located well forward in the body 

cavity. Posteriorly, each Mullerian duct is slightly enlarged to form a short uterus, 

which mostly opens independently into the cloaca. In certain toads, the oviducts unite 

before entering the cloaca by a common orifice. The uteri in most amphibians serve as 

temporary storage places for ova. The oviducts have a glandular lining and prior to 

the breeding season these become greatly enlarged and coiled and secrete a clear 

gelatinous substance. As the eggs pass down the oviducts, several layers of this jellylike 

material are deposited about each ovum. This swells when the egg enters the 

water. 

External fertilization takes place in most anura. The male grasps the female in a 

process called amplexus, and as the eggs emerge from the cloaca, spermatozoa are 

shed over them. No copulatory organs are present. However, each surviving 

amphibian group also contains ovoviviparous individuals in which eggs are retained 

and development proceeds partly in the oviduct. For example, Nectophyrnoides 

produces well-yolked eggs but larvae are retained in the oviduct till metamorphosis is 

completed. 

Internal fertilization occurs in most urodeles, but no copulatory organs are present. 

Males deposit spermatophores, which are small packets of spermatozoa held together 

by secretions of the cloacal glands. A complex mating ritual performed by the male 

stimulates the female, the latter picks up the packet of sperms by muscular 

movements of the cloacal lips. The spermatheca, which is a dorsal diverticulum of the 

cloaca serves as the receptacle for the spermatozoa, which are available for fertilizing 

49

the ova as they pass down the oviducts to the cloaca. Fertilization is external in 

Cryptobranchus, Asiatic land salamanders, and also in the family Sirenidae. A few 

salamanders are ovoviviparous. For example, larvae of Salamandra remain in the 

parent tract to derive nutrition after depletion of egg yolk. They also possess long 

plume like external gills during their oviducal existence, which are shed before birth. 

Internal fertilization occurs in most caecilians. The eversible cloaca of the male is 

considered to serve as the copulatory organ. Caecilians are oviparous or 

ovoviviparous, with some forms retaining the developing embryos in the oviduct and 

feeding on its lining. 

Salamanders show many fascinating examples of physiological adaptations. Although 

most salamanders undergo complete metamorphosis, but some of them also undergo 

the process of incomplete metamorphosis referred to as paedogenesis. A classic 

example of paedogenesis is shown by the Mexican Axolotl, which often breeds in the 

larval state (neotenous forms). On the other hand, the experimental administration of 

thyroxin will cause the larva to loose its gills, develop lungs and emerge from the 

water in an adult-like form. Reducing the water level in which the larva lives, thus 

making gill respiration difficult and facilitating respiration by lungs, can also induce 

metamorphosis. Yet another case of paedogenesis is seen in the Alpine newt, in which 

complete metamorphosis occurs in the habitats of French and Italian lowlands, 

whereas the race that inhabits the colder Lombardy lakes is often neotenous. 

However, among these perennially larval forms no known experimental manipulation 

will induce metamorphosis. 

V. PARENTAL CARE IN AMPHIBIA 

Several remarkable instances of parental care are known among the amphibians (Figure 22). 

A number of different species of frogs and toads construct nests or shelters of leaves or other 

materials in which the eggs are deposited and the young are developed. 

Parental care falls under two heads, which may be found combined in some forms: Firstly 

protection by the parents, either by means of nests or nurseries, or by direct nursing; 

Secondly, by shortening of the metamorphosis period. 

50

(i) Order Anura 

a. Protection by nests and nurseries 

A. In enclosures in the water- Brazilian frog, Hyla faber, protects its progeny by building a 

basin-shaped nursery in the shallow water on the border of a pond (Figure 22 A). The female 

scoops mud to a depth of 3-4 inches and with the material thus removed, a circular wall is 

built that emerges above the water surface. The inside wall is smoothened by webbed, 

flattened hands while bottom is leveled by belly and feet. The eggs and larvae are thus 

protected from attack of many insects and fishes at least for some time, thereafter, heavy 

rains destroy the wall and the larvae directly go to the water. 

B. In holes near water- A still better mode of protecting the offspring during the early stages 

of development has been adopted by the Japanese tree frog, Rhacophorus schlegelli. The 

male and female together bury themselves in the damp earth on the edge of a ditch near a 

flooded rice field, and make a hole or chamber, a few inches above the water level (Figure 

22 B). The walls of this chamber are polished and during this process, the gallery by which 

the frogs had entered into this chamber gets obliterated and the oviposition begins. The 

female first produces a secretion from the cloaca, which is beaten into froth. The eggs are 

deposited into the froth and are fertilized by the male. Thereafter, the parents make an exit 

gallery towards the ditch. It runs obliquely downwards towards the water and is later on used 

by the newly hatched larvae that come to the water to complete their development. 

C. In nests on trees or on rocks overhanging the water- Some tree frogs like the South 

American Phyllomedusa, the Indian Rhacophorus malabaricus and the African Chiromantis, 

deposit their spawn on trees within nests of froth attached to one or many leaves stuck 

together, and overhanging a pool. The larvae move about in the froth and after loosing their 

external gills, fall in water to complete their metamorphosis 

D. In transparent gelatinous bags in water- Phrynixalus biroi has large eggs that are enclosed 

in a sausage shaped transparent common membrane, secreted by the female and this bag like 

structure is left in the mountain streams. The entire development takes place within the eggs 

and the young ones go out in perfect condition. No gills have been observed and the large tail 

serves as the breathing organ of the larva. 

E. On trees or on moss away from water- In several species of tropical American genus 

Hylodes, the eggs are deposited in damp places under stones or on moss or plant leaves, and 

are of large size. The metamorphosis is hurried up within the egg. There is plenty of yolk 

within the egg and hence the entire development occurs there. The young frog leaps out as an 

air breather with a vestige of a tail, which was fully developed and vascularized earlier and 

had served as a respiratory organ. No gills have been observed in the larva. 

b. Direct nursing by the parents 

A. Tadpoles transported from one place to another by the male parents- Small South 

American frogs Phyllobates and Dendrobates and Arthrolepis seychellensis carry well 

developed tadpoles on their back (Figure 22 I). The young ones adhere by their sucker like 

lips and flattened stomachs. They are thus carried from one place to the other effectively. 

B. Eggs protected by the male parent who covers them with his body- The eggs of 

Mantophryne robusta are strung together by an elastic gelatinous envelope. These are around 

15-20 in number, and form a cluster over which the male sits, holding it with both hands. The 

development takes place in this position. The larvae have no gills, but have a large tail, which 

is vascular and respiratory. 

51

C. Eggs carried by the parents- 

I. Round the eggs by the male: In Alytes obstetricans, pairing and oviposition occur on the 

land, and the eggs are deposited by the female in batches of 2-3, at short intervals. The male 

then binds the eggs into a string like structure and wraps them around its legs for protection 

(Figure 22 D). 

II. On the back of the female: 

(i) Exposed- In a Brazilian tree frog, Hyla goeldii, it is the female which takes charge of the 

eggs, carrying them on her back (Figure 22 E). 

(ii) In cell-like pouches- In Pipa Americana and Pipa dorsigera, the eggs are carried on the 

back of the mother, the skin thickens and grows around the eggs until each is enclosed in a 

dermal cell, which is finally covered by a lid, believed to be formed by a secretion from the 

skin glands (Figure 22 G). The eggs are about 100 in number, they develop in these pouches 

and the young leap out in a perfect condition. Pipa is aquatic and pairing occurs in water. 

During egg laying, the cloaca projects as a bladder-like pouch directed forwards between the 

back of the female and the breast of the male. It is by means of this ovipositor, that the eggs 

get evenly distributed over the whole back of the female. However, the method by which 

these eggs get fertilized is not very clear. 

(iii) In a common pouch- In Nototrema, the entire brood is sheltered in a common pouch 

(Figure 22 F), which develops only during the breeding season. How the eggs are introduced 

into the pouch is still unknown, the opening of this pouch is small and located on the 

posterior part of the back. 

III. Exposed on the belly of the female: The female of Rhacophorus reticulatus carries its 

eggs on the belly, which bears shallow impressions when the eggs are removed. 

IV. In the mouth or gular pouch: 

(i) By the male- A remarkable mode of nursing is shown by Rhinoderma darwini. It shelters 

around 10-15 young ones in the gular pouch, which is a modified vocal sac, and development 

is completed here (Figure 22 H). 

(ii) By the female- The female of Hylambates breviceps carries the eggs in her mouth. These 

eggs are large and few in number. 

D. Viviparity- In East African toads, Pseudophryne vivipara and Nectophryne tornieri, larvae 

are found in the uteri. 

ii) Order Urodela 

Urodeles show courtship of various types. During courtship, the male deposits the 

spermatophores attached to the ground or on to the stones, and the female takes them up by 

applying her cloaca on these spermatophores or else by pressing the spermatophores between 

its legs. There are also certain forms as for example, Cryptobranchus, in which fertilization is 

external. Thus, courtship may or may not involve copulation/amplexus: 

1. No amplexus but a lengthy courtship occurs in water. Males have dorsal and caudal crests 

and are more brightly colored than the females. Examples are Molge cristata and Molge 

vulgaris. 

2. Amplexus occurs; however no marked sexual differences of color are found, also neither 

sex shows presence of dermal ornamental appendages. This type is further of two types: 

52

a. Amplexus of a short duration occurs partly or entirely on land, as seen in Salamandra, 

Plethodon and Autodax. 

b. Amplexus of a more or less lengthy duration occurs in water, as found in Molge torosa and 

Molge montana. 

In some forms, the eggs are small and the larvae come out soon and no parental care is seen, 

but in other examples, parental care is found to be as prominent as in anurans. 

a. Protection by nests and nurseries 

A. In holes on land or in trees- Autodax lays about 10-20 eggs in a dry hole in the ground or 

in a hole on a tree, roughly up to 30 feet above ground. The mother or both the parents 

remain in the nest during development to defend the brood and also to provide them with 

moisture. The young ones remain in the nest for a considerable period with the parents. 

B. In a transparent bag in water- Salamandrella keyserlingii deposits its eggs in a gelatinous 

bag, which is attached at one end to an aquatic plant just below the water level. This bag is 

more or less sausage shaped and contains approximately 50-60 eggs. The larvae remain 

within the bag and hatch out at an advanced state of development. 

b. Direct nursing by the parents 

A. Female parent coils around the eggs- In Plethodon, the eggs are laid beneath stones, in 

small clumps, and the mother coils its body around them. The larvae survive on large, 

spherical mass of yolk and do not leave the gelatinous egg capsule until after the loss of the 

gills. Thereafter, the larvae come out. 

B. Male coils around the eggs- In Megalobatrachus maximus, it is the male parent that coils 

around the eggs and protects them during the early stages of development. 

C. Female parent carries the eggs on the back or around the legs- In Desmognathus fusca, the 

eggs are laid in the form of rosary-like strings. The egg strings are then bound around the 

body many times and the female parent nourishes them for some time. 

D. Viviparity- Salamandra maculosa pairs on land, and several months later, the female goes 

to the water and gives birth to 10-50 young ones of small size and similar to the newt larvae 

with their fore limbs developed. In Salamandra atra, the young are retained in the uterus, 

until the completion of metamorphosis. 

(iii) Order Apoda 

In Ichthyophis glutinosa, the female digs a hole close to the surface in damp ground, near the 

water. It then deposits about a dozen large, yellow eggs, measuring 8-10 mm in diameter, and 

coils its snake-like body around them (Figure 22 C). The mother thus, protects the eggs 

against enemies and also against desiccation. Another genus of Apoda, known as Dermophis 

thomensis is viviparous. 

LIST OF REFERENCES CONSULTED 

1. Kardong, K.V. Vertebrates: Comparative anatomy, function and evolution. Third 

Edition. Tata McGraw-Hill Publishing Company Limited. New Delhi 

2. Kent, G.C. and Carr, R.K. Comparative anatomy of the vertebrates. Ninth Edition. 

McGraw-Hill Higher Education (A Division of the McGraw-Hill Companies) 

3. Pough, F.H., Janis, C.M. and Heiser, J.B. Vertebrate life. Sixth Edition. Pearson 

Education 

53

4. Walter, H.E. and Sayles, L.P. Biology of the vertebrates. Third Edition. Khosla 

Publishing House, New Delhi 

5. Weichert, C.K. and Presch, W. Elements of chordate anatomy. Fourth Edition. Tata 

McGraw-Hill Publishing Company Limited. New Delhi 

6. Parker, T.J. and Haswell, W.A. Textbook of Zoology: Vertebrates. Seventh Edition. 

CBS Publishers and Distributors. New Delhi 

7. Young, J.Z. The life of vertebrates. Third Edition. ELBS Funded by the British 

Government 

8. Tomar, B.S. and Bhatnagar, M.C. Comparative osteology. Second Edition. Emkay 

Publications, Delhi 

9. Schmalhausen, I.I. The origin of terrestrial vertebrates. Academic Press Inc. New 

York and London 

10. Prasad, S.N. A Textbook of vertebrate zoology. Sixth Edition. Kitab Mahal, 

Allahabad 

11. Noble, R. C. Biology of the amphibia. Dover Publications Inc. New York 

12. Gupta, R.C. and Chopra G. Comparative anatomy of chordates. R. Chand and Co. 

New Delhi 

13. Rastogi, V.B. A manual of practical vertebrate zoology and physiology. Fifth Edition. 

Kedar Nath Ram Nath, Meerut and Delhi 

14. Verma P.S. A manual of practical zoology: chordates. First Edition (Reprint 1999). S. 

Chand and Co. New Delhi 

15. http://www.zo.utexas.edu/research/salientia/salientia.html 

16. http://tolweb.org/tree?group=terrestrial_vertebrates 

17. http://en.wikipedia.org/wiki/Lissamphibia 

18. http://www-biol.paisley.ac.uk/biomedia/text/txt_amphib.htm 

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Animal Diversity: Chordata

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