Marine, Fresh water, and terrestrial. Bilateral symmetry. A hollow nerve tube on the dorsal side of the body; a stiff notochord beneath it (may be lost or replaced during development of the individual). Several pairs of slits in the "throat" region (pharyngeal slits) may be changed or lost during development of the individual. Some degree of segmentation, especially in arrangement of muscles.
* Subphylum Tunicata (Urochordata). (L. Tunica, Tunic) Larvae free-swimming marine. Notochord and part of nervous system usually degenerate in adults; usually attached. About 700 species. Examples: sea squirts, tunicates.
* Subphylum Cephalochordata. (Gr. Kephale, head and chorde, string of a musical instrument). Free-swimming, translucent, marine. A well-developed hollow dorsal nerve cord, notochord,and pharyngeal slits in the adult. About 28 species. Examples: lancelets, Amphioxus.
* Subphylum Vertebrata. (L. vertebra, joint). Notochord replaced by a "back-bone" of vertebrae during development. An enlarged anterior end of the nerve cord (brain) is protected by cartilage or bone. Most have appendages in pairs.
* Class Aganatha. (Gr. a-, without + gnathos, jaw) No jaws; no paired fins: skeleton of cartilage. A two-chambered heart. 60 species. Examples: lamprey, hagfish.
* Class Chondrichtyes. (Gr. chondros, cartilage + ichthyes, fish) Skeleton of cartilage; five pharyngeal slits visible from out-side. Mouth and nostrils ventral. A two-chambered heart. About 625 species. Examples: sharks, rays, skates.
* Class Osteichthyes. (Gr. Osteon, bone + ichthyes, fish) Skeleton of bone, at least in part. Pharyngeal slits covered (not separately visible from the outside). A two-chambered heart. About 21,000 species. Examples: trout, salmon, cod, perch, sunfish, eels.
* Class Amphibia. (Gr. amphis, double + bios, life) Larvae usually aquatic with gills; most adults terrestrial with lungs. Two pairs of appendages (reduced or lacking in some species) without claws. Adults with three-chambered heart. About 2,500 species. Examples: frogs, salamanders, toads.
* Class Reptilia. (L. repere, to creep) Never breathing with gills. Eggs with shell and membranes enclosing water. Two pairs of appendages (reduced or lacking in some species) with claws. Skin with scales. Heart with three chambers, in some cases the third almost completely separated (nearly four). About 6,000 species. Examples: snakes, lizards, turtles, alligators, chameleons, horned "toads).
* Class Aves. (L. Avis, bird) Scales modified into feathers. Eggs as in reptiles but always hard. Fore appendages modified as wings (or apparently absent). Heart with four chambers. About 9,000 species. Examples: robins, sparrows, eagles, crows, ducks, ostriches, kiwis, penguins.
* Class Mammalia. (L. Mamma, breast) Scales modified as hair. Skin glands (mammary glands) secrete milk on which young are fed. Skeleton with fewer bones than in reptiles. Teeth usually of four well-defined types (in contrast to reptiles): incisors, canines, premolars, and molars. Heart completely four-chambered. About 4,500 species.
Subclasses: monotremes, marsupials, placentals