The echinoderms are radially symmetrical, i.e. a number of similar parts which together make up the body are arranged around an axis (Fig. 145). These animals are usually five-rayed. The mouth lies at lower end of the axis of the body. In some echinoderms the radial symmetry is indistinct, and at the same time there is a more or less clear bilateral symmetry (p. 16). The dermal skeleton either consists of numerous microscopic calcareous deposits, or of firm plates; the skin often bears spines, hence the name of the group. Echinoderms move by means of rows of tubular feet, which are connected with a water-vascular system, and are protruded by means of small bladders, connected with the same system, which contract and force water into them. The form of the body is apple-shaped to spherical (=Regular Sea Urchins=), shield- or heart-shaped (=Irregular Sea Urchins=), pentagonal to star-shaped, with arms of varying length (=Starfishes=), or cylindrical to vermiform (=Sea Cucumbers=). All echinoderms are marine, and none of them, therefore, are of agricultural importance, though near the coast starfishes are sometimes used as manure. =Sixth Sub-Kingdom=: =CŒLENTERATA= (ZOOPHYTES). 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. [Illustration: FIG. 146.—Freshwater Polype (_Hydra_), bearing two buds of different ages. Highly magnified. ] [Illustration: FIG. 147.—A Phosphorescent Jellyfish (_Pelagia noctiluca_), ½ the natural size. ] [Illustration: FIG. 148.—A Sea Anemone (_Sagartia nivea_). ]