With a few exceptions the zoophytes are radially symmetrical (p. 252), and four-, six-, eight-, or many-rayed. They consist of a body-wall enclosing a cavity which, in the simplest forms (Fig. 146), is sack-like, but in several of the higher sub-groups divided up, and serves, not only for the reception and digestion of food, but also for the circulation of the digested food. Many zoophytes develop hard parts in the walls of their bodies, which remain after their death. The red coral used for ornamental purposes is of this nature, and so also are the coral islands which polypes build up. With the exception of the sponges, which are usually reckoned as belonging to this sub-kingdom, these animals possess “nettling organs” with which they kill their prey, and by which they are protected from attack. They reproduce by eggs, and most of them by means of budding as well (Fig. 146). In many species the buds are not detached as in the Freshwater Polype (Hydra), but remain attached so that a “colony” is formed. There are also free-swimming cœlenterates, known as jellyfishes or medusæ (Fig. 147). Many of these lay eggs which do not develop into fresh medusæ, but into polypes, which increase by budding to form colonies, on which medusæ are once more developed as buds. There is thus an alternation of generations (p. 16), in which the medusæ represent the sexual and the polypes the asexual stage.
Fig. 146.—Freshwater Polype (Hydra), bearing two buds of different ages. Highly magnified.
Fig. 147.—A Phosphorescent Jellyfish (Pelagia noctiluca), ½ the natural size.
Fig. 148.—A Sea Anemone (Sagartia nivea).
Almost all zoophytes live in the sea, but some few in fresh water; none are important agriculturally. The following belong to this sub-kingdom: 1. The Hydroid Zoophytes (Hydrozoa), including the medusæ of the North Sea, many of which are the sexual stage in the life history of fixed asexual polypes. The common Freshwater Polypes (Hydra viridis, fusca, etc.). 2. The Flower Polypes (Anthozoa), to which belong Red Coral (Corallium rubrum), and the reef-building corals (Astræa, Mæandrina, etc.). 3. The Sponges (Porifera), which are usually considered cœlenterates, though they possess no nettling organs (Freshwater Sponge = Spongilla, Bath Sponge = Euspongia officinalis).