Chapter 34: Phylum Chordata

Phylum Chordata• all chordates are have true tissues (eumetazoans) & a true coelom

and exhibit bilateral symmetry & deuterostome development• 4 derived characters :

• notochord – a long, flexible rod that appears during embryonic development between the digestive tube and the dorsal nerve cord (present in all embryos but may be reduced in adults)

• dorsal, hollow nerve cord – develops into the central nervous system (brain & spinal cord)

• pharyngeal clefts – grooves that separate a series of pouches along the sides of the pharynx

• muscular, post-anal tail

Invertebrate Chordates• chordates that lack a backbone

Subphylum Urochordata• aka: tunicates

• resemble other chordates during larval stage• larvae use tail muscles & notochord to swim• when larvae settle, they undergo metamorphosis & lose many chordate

characteristics• adults filter-feed by bringing water in through a siphon, letting it pass through the

pharyngeal slits into an atrium & out through a second siphon (as a result, tunicates are often called “sea squirts”)

Subphylum Cephalochordata

• aka: lancelets• bladelike shape• adults retain chordate

traits after metamorphosis• first animals to show sign

of a brain:• swollen tip at anterior end of dorsal nerve cord• the cells of this region express the same Hox

genes as the brain of vertebrate embryos

Craniates• chordates with a head

• consists of a brain, eyes, sensory organs, & a skull• derived characters:

• increased genetic complexity (ex: 2 clusters of Hox genes)• neural crest• pharyngeal clefts become gill slits• higher metabolism & more extensive muscular system• heart with 2 chambers• red blood cells & hemoglobin• kidneys

Class Myxini• aka: hagfishes• oldest living craniates• invertebrates• skull made of cartilage• retain notochord in adulthood (used for movement)• has a small brain, eyes, ears, & nasal opening that connects

with pharynx• tooth-like formations• slime glands

Vertebrates• craniates that have a backbone• derived characters:

• additional genetic complexity associated with advancements in nervous system & skeleton development

• more extensive skull• backbone composed of vertebrae that enclose the spinal cord

& have assumed the role of the notochord

Class Cephalaspidomorphi• aka: lampreys• oldest lineage of vertebrates• most are parasites• round, jawless mouth• rasping tongue• cartilage skeleton (contains no collagen)• notochord persists as main axial skeleton but has a

cartilaginous pipe around it• live in marine & fresh water

Gnathostomes• vertebrates that have jaws

• hinged jaws evolved from the modification of skeletal parts that had once supported the pharyngeal (gill) slits

• additional characteristics• 4 clusters of Hox genes• increased gene complexity• enlarged forebrain• lateral line system (in aquatic species) – senses water vibrations• mineralized endoskeleton• appeared in fossil record ~470 mya

Class Chondrichthyes• skeleton composed of cartilage• streamlined bodies• denser than water & sink if stop swimming• 3 methods of reproduction:

• oviparous – young do not develop w/in female; female lays eggs that hatch outside the body

• ovoviviparous – young develop w/in female w/out a placenta for nourishment

• viviparous – young develop w/in female & fed by a placenta• (ex) sharks, skates, rays, ratfishes, chimaeras

Class Osteichthyes

• aka: “bony fishes”• ossified endoskeleton• bodies covered in scales• breathe with gills• swim bladder (to control buoyancy)• mucus secreting glands in skin• lateral line system• most oviparous

2 Subclasses of Bony Fishes

•Subclass Actinopterygii• aka: ray-finned fishes• fins supported by long, flexible rays• fins modified for maneuvering & defense• (ex) bass, trout, perch, tuna, etc.

•Subclass Sarcopterygii• aka: lobe-finned fishes• rod-shaped bones surrounded by a thick layer of muscle in the

pectoral & pelvic fins• (ex) coelacanths & lungfishes

Tetrapods• gnathostomes that have limbs & feet

• additional characteristics:• bones of pelvic girdle fused to backbone• no gill slits• ears – detect airborne sounds

Class Amphibia• amphibian means “2 lives” (aquatic & terrestrial) which is a

reference to the metamorphosis of many species• depend on damp/wet habitats because gas exchange occurs

through the skin• eggs lack a shell & fertilization is external so they depend on

water to complete their life cycle• 3 orders:

•Order Urodela (tailed) – salamanders •Order Anura (tailless) – frogs•Order Apoda (legless) – caecilians

Amniotes• tetrapods that have a terrestrially adapted egg

• amniotic egg: • has a shell that retains water• has extraembryonic membranes that function in gas exchange,

waste storage, & the transport of nutrients to the embryo

Reptiles• waterproof scales (keratin)• rely on lungs for gas exchange• lay shelled eggs on land• internal fertilization• “cold-blooded” (ectothermic) – regulate body temperature by

absorbing heat from the environment (rather than via metabolism)• extinct groups = dinosaurs (land), pterosaurs (air), & plesiosaurs

(marine)• living groups = turtles, tuataras, lizards, snakes, alligators, crocodiles

Birds• lay amniotic eggs• have keratin scales on legs• bodies constructed for flight:

• light, hollow bones• few organs• wings & feathers

• endothermic (regulate body temperature internally)• toothless• acute vision• efficient respiratory & circulatory systems with a 4-chambered heart

Class Mammalia• mammary glands (produce milk for offspring)• hair• endothermic• most born rather than hatched• internal fertilization• proportionally larger brains than other vertebrates• teeth of different shapes & sizes (adapted for different foods)• efficient respiratory & circulatory systems including a 4-

chambered heart

3 Groups of Mammals•Monotremes (egg-laying mammals)

• have hair & produce milk but lack nipples• (ex) platypuses, echidnas (spiny anteaters)

•Marsupials (mammals with a pouch)• embryos begin development in uterus but are born early &

complete development in a marsupium (pouch)• (ex) opossums, kangaroos, koalas, wombats

•Eutherians (placental mammals)• embryos complete development with the uterus & are joined to the

mother by placenta

Primates• hands (opposable thumb) & feet adapted for grasping• large brains & short jaws• forward-looking eyes• digits with flat nails• excellent eye-hand coordination• well-developed parental care• complex social behavior• 3 subgroups:

• lemurs, lorises, & pottos• tarsiers• anthropoids (monkeys, apes, & humans)

Humans

• stand upright & walk on 2 legs (bipedal posture)• large brain• reduced jawbones & jaw muscles• shorter digestive tract• reduced size-difference between the sexes• capable of language, symbolic thought, and the

manufacture & use of complex tools• study of human origins = paleoanthropology