ZOOPHYTES. 253 



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 import- 

 ance, though near the coast starfishes are sometimes 

 used as manure. 



Sixth Sub-Kingdom : C(ELENTERATA (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 coelenterates, known as jellyfishes or 

 medusae (Fig. 147). Many, of these lay eggs which 

 do not develop into fresh medusae, but into polypes, 

 which increase by budding to form colonies, on which 

 medusae are once more developed as buds. There is 

 thus an alternation of generations (p. 16), in which 

 the medusae represent the sexual and the polypes the 

 asexual stage. 



