Summary

The animal kingdom, for all its diversity, can be sorted into an orderly scheme because animals share a limited set of fundamental structural features. A handful of characters — the level of organisation (cellular, tissue, organ, or organ-system), the type of symmetry (radial or bilateral), the number of germ layers (diploblastic or triploblastic), the presence or absence of a body cavity or coelom, the presence of segmentation, and the presence of a notochord — together allow the entire kingdom to be classified. Beyond these broad features, each phylum and class also carries its own distinctive characters.

The simplest phyla show the least complex body plans. Porifera, the sponges, are multicellular animals with a cellular level of organisation and possess characteristic flagellated cells called choanocytes, along with a body walled by pores and canals. The coelenterates (Cnidaria) bear tentacles armed with stinging cells (cnidoblasts) and are mostly aquatic, either fixed in place or free-floating. The ctenophores are marine animals that move using rows of comb plates. In the platyhelminths, the body is flattened and bilaterally symmetrical; the parasitic flatworms carry suckers and hooks for attachment. The aschelminthes, or roundworms, are pseudocoelomates and include both parasitic and free-living forms.

With the higher invertebrate phyla, true coeloms and greater complexity appear. The annelids are metamerically segmented worms with a genuine coelom and a closed circulation. The arthropods are the most abundant animals of all, recognised by their jointed appendages and a hard outer covering (exoskeleton). The molluscs have a soft body usually protected by an external calcareous shell. The echinoderms possess a spiny skin and are distinguished above all by their water vascular system. The hemichordates form a small group of worm-like marine animals with a cylindrical body divided into a proboscis, a collar and a trunk.

Phylum Chordata stands apart because its members possess a notochord, either throughout life or only during early embryonic development. The other shared chordate features are a dorsal, hollow nerve cord and paired pharyngeal gill slits. Within the chordates, the protochordate subphyla lack a backbone, while the subphylum Vertebrata develops a vertebral column. Among vertebrates, some lack jaws — the Agnatha, represented by the class Cyclostomata, the most primitive chordates, which live as ectoparasites on fishes. The remaining vertebrates, the Gnathostomata, possess jaws and are divided into two super classes, Pisces and Tetrapoda. …