CHAPTER 23
Chordates
Integrated Principles of
Zoology, 17th edition
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Figure 23.2
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Figure 23.2
Cladogram of living members of phylum
Chordata.
Figure 23.3
Figure 23.3
Phylogenetic tree of the
chordates.
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Phylum Chordata
• Phylum Characteristics
– Bilateral symmetry
– 5 unique features present at some point
•
•
•
•
•
Notochord
Pharyngeal slits or pouches
Dorsal tubular nerve chord
Endostyle or Thyroid gland
Postanal tail
Five Chordate Hallmarks
• Notochord
– Rod like flexible structure extending the length of the body
– Found in ventral surface of neural tube
– Composed of mesoderm
– Key site of muscle attachment
– Pliable to allow undulation movement
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Five Chordate Hallmarks
• Dorsal Tubular Nerve Cord
– The cord is dorsal to digestive tract and the anterior end enlarges to
form the brain
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Five Chordate Hallmarks
• Pharyngeal Pouches and Slits
– only seen during embryonic stages
– Develop into gills in water-breathers
– In terrestrial tetrapods, the pharyngeal pouches give rise to a variety
of structures: Eustachian tube, middle ear cavity, tonsils, and
parathyroid glands
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Five Chordate Hallmarks
• Endostyle or Thyroid Gland
– The endostyle was not recognized as a chordate characteristic until recently
– Endostyle releases mucus to capture food
– Chordates: binds iodine and produces hormones
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Five Chordate Hallmarks
• Postanal tail
» Developed for propulsion in water
» Modified into caudal fin
» Many tetrapods have well-developed tails for balance
» Coccyx: trace remnant of tail in humans
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Phylum Chordata
• Subphylum Urochordata (Gr. uro, tail + L. chorda,
cord)—Tunicates
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Phylum Chordata
• Subphylum Urochordata Characteristics
– Solitaire or colonial
– Sessile adults
– About 1600 species of tunicates identified; found in all
seas and at all depths
– Attach to solid substrates
– two siphons that permit seawater to circulate through the
body
– Oral (incurrent) siphon also the mouth
– Atrial (excurrent) siphon water being expelled
Figure 23.6
Figure 23.6 Metamorphosis of a solitary ascidian from a
free-swimming larval stage.
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Figure 23.4
Figure 23.4
Structure of a
common tunicate,
Ciona sp.
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Reproduction and Development
• Monoecious
• Self fertilization and cross fertilization
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Phylum Chordata: Subphyla Characteristics
• Subphylum Cephalochordata (Gr. kephalo, head + L.
chorda, cord)—Lancelets
• Elongated laterally flattened nearly transparent
• Size up to 5 cm tadpole like animals
• About 29 species of Amphioxus are described with 5 occurring in
North American coastal waters
• All five chordate characteristics persist throughout life
• Live in Shallow waters
• genera Branchiostoma (Amphioxus)
• Filter-feeders that burrow into sand with anterior end out
Phylum Chordata
• Subphylum Cephalochordata Characteristics:
– Notochord extends from head to tail
– Oral Hood: anterior end of body
• Cirri: finger-like projections that keep particles
out of mouth and help in feeding
Subphylum Cephalochordata
Phylum Chordata
• Subphylum Cephalochordata Characteristics:
– Feeding
• Endostyle: binds iodine and produces mucus
that traps food as it passes into atrium
• Food-bearing mucus passes into short
esophagus
• Gut extends as elongated intestine opening just
at anus just before caudal fin
• Hepatic Cecum: lipid and glycogen storage
Phylum Chordata
• Subphylum Cephalochordata Characteristics:
– Circulatory system:
• Closed Circulation similar to primitive fish
• No heart; ventral aorta pumps blood through
branchial arteries to dorsal aortas to body back
to veins
• Mainly carries nutrients
– Gas exchange:
• Mainly occurs across skin
– Excretion:
• Protonephridia accumulate nitrogen waste and
take to atrium via nephridioduct
Phylum Chordata
• Subphylum Cephalochordata Characteristics:
– Reproduction
• Dioecious but can’t tell male from female
• From 25 to 38 pairs of gonads along atrial wall
• Spawning: release egg and sperm by rupturing
atrial wall
• Free-swimming bilateral larvae eventually settle
into sand and metamorphose into adults
Phylum Chordata
• Paedomorphosis retention by an organism of juvenile
or even larval traits in the adult form.
Phylum Chordata
• Subphylum Vertebrata (Craniata)
– Common Characteristics
• Well developed coelom segmented into:
–Pericardial cavity: containing the heart
–Pleuroperitoneal cavity: containing lungs
and abdominal organs
• Body covering (skin): stratified epithelium from
ectoderm with inner connective tissue from
mesoderm; numerous variation
Phylum Chordata
• Subphylum Vertebrata
– Common Characteristics
• Bone or cartilage endoskeleton with vertebral
column and a head skeleton
• Appendicular skeleton with two pairs of
appendages supported by girdles (pectoral and
pelvic)
• Digestive tract of muscles ventral to vertebral
column
Phylum Chordata
• Subphylum Vertebrata
– Common Characteristics
• Circulatory system of ventral, multichambered
heart attached to arteries, veins, and
capillaries; blood contains variety of cells
suspended in plasma
• Excretory system with pair of tubular kidneys
featuring waste-draining ducts
• Endocrine system of glands without ducts
located randomly
• Large three-lobed brain; 10 to 12 cranial nerves
and paired sense organs
Neural Crest
• Neural crest cells are a
temporary group of cells that
arise from the embryonic
ectoderm cell layer, and in turn
give rise to a diverse cell
lineage—including
melanocytes, craniofacial
cartilage and bone, smooth
muscle, peripheral and enteric
neurons and glia.
Neural Spines
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Musculoskeletal Modifications
Physiology Upgrade
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New Head, Brain, and Sensory Systems
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The Earliest Vertebrates
– Ostracoderms were armored jawless fishes, which were the earliest known
vertebrate fossils found in the late Cambrian deposits
– These show many vertebrate characteristics like heart, pair eyes, ear capsules,
and rudimentary vertebrae
– Earliest Ostracoderms had armor with bone in dermis but lacked paired fins
that later fishes used for stability
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Placoderms and Acanthodians
– One of the earliest jawed vertebrates are called Placoderms that
appeared in the early Silurian and were heavily armored and many
were grotesque
• Had diamond shaped scales and large plates of bone but went extinct and
left on descendants
– Acanthodians are another jawed bony group that underwent a great
diversification into bony fishes that dominate waters today
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Gnathostomes
• All jawed vertebrates are called Gnathostomes whether living
or extinct
– Gnathostomes constitute a monophyletic group since the presence of
jaws is a derived character state shared by all jawed fishes and
tetrapods
– Agnathans, defined by the absence of jaws, is paraphyletic since many
vertebrate ancestor groups lack jaws
– Having jaws was one of the most important events in vertebrate
evolution as this allowed predation on large and active food
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Evolution of Jaws
– Evidence indicates jaws arose from the modification of 1st two cartilaginous
gill arches
• The gill arches and jaws form from upper and lower bars that bend forward and are
hinged in the middle
• Both gill arches and jaws are derived from neural crest cells rather than from
mesoderm
• Jaw musculature is homologous to the musculature that originally supported gills
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