Animal Diversity: Chordata

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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
Page 1 and 2: List Of Contents I. Introduction An
Page 3 and 4: Tetrapoda The class Amphibia is com
Page 5 and 6: II. CLASSIFICATION Basal tetrapods
Page 7 and 8: Microsaurs represent a diverse grou
Page 9 and 10: espiration. Amphiuma is an elongate
Page 11 and 12: latter living on land have a warty
Page 13 and 14: Family Character BRACHYCEPHALIDAE V
Page 15 and 16: Family Character Rhacophoridae Very
Page 17 and 18: mineralized tissue in their mesoder
Page 19 and 20: available led to an extensive remod
Page 21 and 22: Devonian. This is particularly mark
Page 23: The median fin is completely lost a
Page 28 and 29: Respiratory movements involve the a
Page 30 and 31: heart they are separated by a peric
Page 32 and 33: capillaries. The ventral branch cal
Page 34 and 35: Skull of frog (Figure 12) 1. Skull
Page 36 and 37: Typical second vertebra 1. In frog
Page 38 and 39: 10. The sternum lies in the mid-ven
Page 40 and 41: Hind-limbs bones of frog (Figure 17
Page 42 and 43: diencephalon with the medulla oblon
Page 44 and 45: tympanic membrane or tympanum. The
Page 46 and 47: kidney with peritoneal connections
Page 48 and 49: desert-dwelling frogs like Cycloran
Page 50 and 51: the ova as they pass down the ovidu
Page 52 and 53: C. Eggs carried by the parents- I.
Page 54: 4. Walter, H.E. and Sayles, L.P. Bi

Animal Diversity: Chordata

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