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