| A | B |
|---|
| vertebrates | chordate animal with a backbone: mammals, birds, reptiles, amphibians and various classes of fishes |
| chordates | member of a diverse phylum of animals that possess a notochord, a dorsal, hollow nerve cord; pharyngeal gill slits; and a postanal tail as embryo |
| notochord | longitudinal, flexible rod formed from dorsal mesoderm and located between the gut and the nerve cord in all chordate embryos |
| urochordates | chordate without a backbone, commonly called a tunicate, a sessile marine animal |
| tunicates | a group of underwater saclike filter feeders with incurrent and excurrent siphons that is classified within the phylum Chordata |
| lancelets | the modern representatives of the subphylum Cephalochordata, usually said to be the sister group of the craniates |
| cephalochordates | chordate without a backbone, represented by lancelets, tiny marine animals |
| paedogenesis | precocious development of sexual maturity in a larva |
| neural crest | band of cells along the border where the neural tube pinches off from the ectoderm; the cells migrate to various parts of the embryo and form the pigment cells in the skin, ones of the skull, the teeth, the adrenal glands, and parts of the peripheral nervous system |
| tetrapods | vertebrate possessing two pairs of limbs, such as amphibians, reptiles, birds, and mammals |
| amniotic egg | shelled, water-retaining egg that enables reptiles, birds, and egg-laying mammals to complete their life cycles on dry land |
| amniotes | vertebrate possessing an amnion surrounding the embryo; reptiles, birds, and mammals are amniotes |
| superclass agnatha | a superclass of jawless fish in the phylum Chordata, subphylum Vertebrata. The group excludes all vertebrates with jaws, known as gnathostomes. |
| ostracoderms | extinct agnathan, a fishlike creature encased in an armor of bony plates |
| superclass gnathostomata | he group of vertebrates with jaws. Gnathostome diversity comprises roughly 60,000 species, which accounts for 99% of all living vertebrates |
