Chapter 34: Phylum Chordata

1. Chapter 34: Phylum Chordata

2. 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

3. Invertebrate Chordates • chordates that lack a backbone

4. 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”)

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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

14. Tetrapods • gnathostomes that have limbs & feet • additional characteristics: • bones of pelvic girdle fused to backbone • no gill slits • ears – detect airborne sounds

15. 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

16. 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

17. 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

18. 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

19. 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

20. 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

21. 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)

22. 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