This is the longest chapter at 72 pages. There isn't a biological classification that includes all the invertebrates. They are just all the reef animals that are not vertebrates, in other words, not fish, including all the various diverse groups on the reefs that do not have backbones. The chapter begins with a simplified taxonomy of reef organisms, in the form of a chart showing the place of the various phyla that live on the reef in the animal kingdom, and a discussion of the way the field of taxonomy works. Then, several examples from different groups are presented, beginning with cnidarians other than the stony corals, first the hydrocorals(4), then alcyonaceans, or soft corals (9), and antipatharians (1). Four species of anemones are looked at, along with two commensal shrimp and a commensal fish. Corallimorphs (2), zoanthids (2) cerianthus (1), and a scyphozoan (1) are all additional cnidarian types described. Sixteen types of sponges are described and shown, many as full-page figures, followed by 7 polychaete worms. The next groups shown are 5 molluscs, 7 crustaceans, and 17 echinoderms of various kinds, organized by group. The chapter ends with one tunicate, an apparent invertebrate as an adult that is put with the chordates because of its larval relation to the vertebrates. All of these diverse groups are described in terms of their similarities to others within the group and differences from other groups, and their distinct ways of living and feeding are discussed. Most are shown by a single, often large, photo, while a few have multiple photos showing their variety in coloration or morphology. For each species the scientific name is given, but no common names are used, because they are often imprecise and most of the species shown do not have them.