Ch 23: Phylum Chordata - PorterMax

Z203/ Unit 1
Ch 23: Phylum Chordata
I. Characteristics
­ Approximately 45,000 species in 8 major classes
A. Structural Plan
­ Shared features w/ Invertebrate Phyla:
bilateral symmetry, anterioposterior axis, eucoelom,
metamerism, cephalization, complete digestion
­ Echinoderm/Hemichordate assemblage may be sister
Groups that share:
Deuterostomes =
mouth 2 o derived
coelom formed from
pouches in
mesoderm
­ name CHORDATA comes from Notochord
(rod­like, semi rigid tissue enclosed in connective tissue)
­ Notochord extends length of body & lies between gut
tube & nervous system
­ serves to stiffen body or act as
skeletal axis
­ Chordates have more structural
unity than other phyla, share 5
hallmark characteristics plus
genetics
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II. Five Chordate Hallmarks
A. Notochord
­ always found at some embryonic stage of all chordates
­ 1 st part of endoskeleton to develop
­ serves as axis for muscle attachment
­ may persist throughout
life (protochordates &
jawless vertebrates) or
be lost completely w/
metamorphosis
­ series of cartilaginous bony vertebrae form in
vertebrates derived from mesoderm cells lateral
to notochord
­ most vertebrates, notochord displaced by vertebrae
(remains persist as intervertebral discs)
B. Hollow Dorsal Nerve Cord (Tubular)
­ single cord dorsal to alimentary canal
(ventral in invertebrates)
­ anterior end enlarges to form brain
­ cord produced by infolding of ectoderm on dorsal side
C. Pharyngeal Pouches & Slits
­ slits lead from pharyngeal cavity to outside
­ form by inpocketing of ectoderm &
outpocketing of pharynx endoderm
­ 2 pockets join in aquatic chordates & form
complete slit for filter feeding & respiration
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­ grooves persist (pockets don’t join) in amniote chordates
(give rise to structures: eustachian tubes, middle ear cavity,
tonsils, & parathyroid glands)
D. Endostyle
­ mucous secreting structure found in the Pharynx floor
(traps small food particles)
­ found in Protochordates & Lamprey larvae
­ cells associated secrete iodine­based proteins (Thyroid
gland in Vertebrates)
E. Postanal Tail
­ works with muscles (Myomeres) & notochord to provide
motility & stability
­ Aids in propulsion in nonvertebrates & fish but vestigial in later
lineages
III. Ancestry & Evolution
A. History
­ earliest protochordates soft bodied (not fossilized)
­ deuterostomes are natural grouping w/ common origin
in Precambrian Seas
(Echinoderms, Hemichordates, Chordates)
­ anatomical, developmental, & molecular evidence
indicates chordates arose 570mya from lineage
related to Echinoderms & Hemichordates
­ significant developments occurred within chordates:
skull, jaw, amnion, fins & limb arrangement
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­ group divided into 3 Subphylum
1. Urochordata
2. Cephalochordata
3. Chordata
INVERTEBRATE CHORDATES:
IV. Subphylum Urochordata
"Tunicates"
“Tail Chordates”
A. General Characteristics
­ 2,000 species; occur in all seas & at all depths
­ free living larva, most adults metamorphose into sessile
­ only larvae bears all chordate hallmarks; most adults
only retain a few
EXTERNAL:
­ tunic (tough nonliving test produced by mantle)
surrounds body that contains Cellulose
­ adults form has sac­like body with stolons at the base and
siphons at the top (may have more than 2 & colonial forms
merge their siphons:
Incurrent siphon has sensory tentacles
Excurrent siphon has pigment
spots
INTERNAL:
­ flow pattern of water functions in
respiration & digestion & excretion
(incurrent siphon→pharynx→gill
slits→aterial cavity
→excurrent siphon)
­ Respiration occurs across pharyngeal gill slits
­ Digestion – extracellular complete using mucous nets secreted
by endostyle to capture food particles
(incurrent siphon→pharynx→gill slits→esophagus
→stomach→intestine→anus→atrial cavity→
excurrent siphon)
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­ Open circulatory system with ventral heart + 2 large blood
vessels (pumps 2 directions)
* unique blood characteristics (RBC without hemoglobin)
* blood flow through gill slits aids in respiration & excretion
­ Nervous system – reduction of dorsal nerve cord & ganglia in
adult (sessile way of life)
­ notochord & tail disappear in metamorphosis
­ Reproduction – dioecious with larva free
swimming (larva retain chordate features)
­ 3 Groups:
Ascidians "sea squirts"
Thalacians "salps"
Larvacea "larval tunicates"
(paedomorphic)
V. Subphylum Cephalochordata "Lancelets"
“Head Chordates”
A. General Characteristics
­ 29 species worldwide, 5 species in North American
­ live in sandy bottom of coastal waters
EXTERNAL:
­ slender, laterally flattened, translucent, 5­7cm
­ reduced fins each with a fin­ray aid in movement & digging
(dorsal, caudal, ventral)
­ fusiform body shape (tapered anterior & posterior)
*Amphioxus = “both ends” “sharp”
­ anterior end has rostrum, oral hood,
oral tentacles @ mouth
INTERNAL:
­ retain the 5 distinct chordate
characters in simple form
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­ Respiration – flow pattern of water across gill slits
­ Digestion – Complete system with cilia (wheel organ) driving
currents for filter feeding (like tunicates)
*Endostyle located at floor of Pharynx
*Cecum resembles vertebrate pancreas (secretes enz)
(tentacles→wheel organ→mouth→pharynx→gill slits
(endostyle)→stomach (hepatic cecum)→intestine→anus)
­ Trunk muscles suggest vertebrate pattern (myomeres)
­ Circulatory ­ same basic Closed circulatory plan as vertebrates
*no heart – modified arteries for pumping
*capillaries in gill slits increase gas exchange for respiration
* blood only carry nutrients ­ no RBC or Hg for O 2
­ Excretory ­ several U­shaped Nephridia (exit atriopore)
­ Nervous system consists of ganglia + dorsal nerve cord &
spinal nerves
* simple senses include tactile & chemoreceptors
* ocelli (eye spots) for light detection
­ Reproduction – Dioecious with gonads ventral to pharynx
* gametes exit atriopore
VI. Subphylum Vertebrata
“Craniata”
A. General Characteristics
EXTERNAL:
1. Cephalization
­ predominance of head and its development drive distinct
bilateral symmetry & nervous system advancements
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2. Paired Limbs
­ pectoral & pelvic appendages originate as swimming
stabilizers
­ jointed limbs suited for terrestrial life & permit fine movement
INTERNAL:
1. Living Endoskeleton
­ internal location departs from invertebrate pattern
­ grows w/ body, not confining nor molted
­ greater economy of building materials
­ provides jointed scaffolding for muscles; muscles
cushion it from impact
­ skull & ribcage enclose & protect vital organs
­ bony plates , tough integument protects (exoskeleton)
­ cartilage likely 1 st endoskeletal material
*superior to bone for fast growth)
­ cartilage remains 1 o endoskeletal material for
Agnathans & sharks
­ bones offer advantage of storing minerals (P,Ca) &
mechanical strength for terrestrial life
2. Physiology – system become more complex
­ Respiration – perforated pharynx originated for filter feeding
* evolved to a muscular pharynx for pumping water
* shifted function from feeding to respiratory (gills or lungs)
­ Digestion – Complete system with more specialization
* accessory digestive glands (liver & pancreas) needed
for larger food processing
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­ Circulatory – Closed system w/ multiple chambered hearts and
blood components
* RBC with hemoglobin aid in
O 2 dispersal
* Aortic arches dedicated to
gill slits for gas
exchange
­ Excretory ­ paired kidneys
remove metabolic waste
products
3. Advanced Nervous System
­ switch to active predation favored sensory, motor &
integrative controls
­ anterior end of nerve cord became tripartite brain
(forebrain, midbrain, hindbrain)
­ selected for sensory organs:
* paired eyes w/ lenses &
inverted retinas,
* paired ears,
* receptors (pressure, electro &
chemo)
­ head & paired sense organs
possible due to derivatives of
ectoderm germ layers:
*neural crest (ectodermal cells) develops into
structures (cranium, teeth, ganglia, endocrine
glands, & some nerves)
*epidermal placoides (plate like ectodermal
thickenings) give rise to olfactory r/c, eye lens, inner
ear epithelium, & receptors (taste, photo & lateral
line mechano)
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­ significant changes in genetics (duplication of Hox genes)
drove changes in nervous system and entire body
*1 copy of Hox genes in Amphioxus
* 4 copies in Gnathostomes
VII. Evolutionary History
A. Ancestral Invertebrate Chordates
­ ribbon­shaped fish from Mid­Cambrian are oldest
invertebrate Chordate fossils seen
(similar to lancelets)
­ Garstang's Hypothesis of Chordate Larval Evolution (1928)
Paedomorphosis: larvae failed to metamorphosis into adult
form, but developed gonads & reproduced in larval stage
­ Lancelets diverged early from shared common ancestor
w/ tunicates & many consider them sister group & closest
living relative to vertebrates
­ Lamprey larva (ammocoetes) were once believed to be
adult lancelets due to similarities
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B. Earliest Vertebrates
1. Superclass Agnatha: Jawless Ostracoderms
articulated fossils found from late Cambrian (530mya)
heavily armored (bones in dermis) extinct fish
(lasted 150mya)
lacked lateral fins for stabilization
rudimentary vertebrae
fixed circular mouth
2. Superclass Agnatha: Modern Jawless Fish
naked hagfish and lampreys
classification based on missing trait ­ paraphyletic
3. Gnathostomes: Early Jawed Vertebrates
all have jaws = modifications of 1 st two cartilaginous
gill arches derived from neural crest (not
mesoderm like most bones)
jaw musculature homologous to gill musculature
most likely monophyletic group
Placoderms (early Devonian)
among 1 st jawed vertebrates
armored fish covered w/ diamond shaped scales
or large plates
great diversity arose but went extinct by
Devonian period
grew to 10m length
Acanthodians
gave rise to modern bony fish
4. Modern Fish & Tetrapods
ancestry uncertain
unknown from which chordate group vertebrate
lineage originated
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C. Classification of Phylum Chordata
Protochordata (Acrania)
Subphylum Urochordata (tunicates)
Subphylum Cephalochordata (lancelets)
Craniata
Subphylum Vertebrata
Superclass Agnatha
Class Myxini (hagfish)
Class Pteromyzontida (lamprey)
Superclass Gnathostomata
Class Chondrichthyes
Class Actinopterygii
Class Sarcopterygii
Class Amphibia
Class Reptilia
Class Aves
Class Mammalia
(sharks, rays, skates)
(ray­finned fish)
(lobe­finned fish)
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