Scientists often categorize animal phyla into about thirty-five divisions—whether they are found in the oceans, on land, or in between. The nine major phyla include Porifera (sponges), Cnidaria (hydra and jellyfish), Mollusca (mollusks), Echinodermata (starfish and sea urchins), Nematoda (roundworms), Annelida (segmented worms), Platyhelminthes (flatworms), Arthropoda (insects, crustaceans, and arachnids), and Chordata (fish, amphibians, reptiles, birds, and mammals).

All chordates—which include fish, amphibians, reptiles, birds, and mammals—share several features, the major ones being a notochord, dorsal nerve cord, and pharyngeal gill pouches. The notochord, a supporting rod made of cartilage, runs along the dorsal part of the body. It is always found in embryos, but in most vertebrates it is replaced during late embryonic development by a backbone of bony or cartilagelike vertebrae. The tubular dorsal nerve cord, near the notochord, is also formed during development of the embryo. In most vertebrates, the nerve cord eventually becomes a hollow cord and is protected by the backbone. The pharyngeal gill pouches appear during embryonic development on both sides of the throat region (the pharynx), but in some species, it does not develop. In human embryos, these gill pouches show a series of folds (thus the word pouches) that look similar to those of early fish embryos. But while in fish they would develop into gills, in humans (and other mammals) they develop into the ear’s Eustachian tube and middle ear, tonsils, parathyroid, and thymus.