MANUAL OF NATURAL HISTORY. 343 



tra, and some among them are even free, as the 

 fresh-water genus Cvistatella. In some forms the 

 cells are very thin and pellucid ; in others they are 

 horny or membranous ; while many again inhabit 

 cells of a hard calcareous material. The general 

 form of the polyzoary is susceptible of very great 

 variation, from the broad palmate expansion of the 

 Flustra foliacea to the lobate fleshy Alcyonidium; 

 from the long tubular cells of the Tubulipora to the 

 elegant, jointed Eucratea; from the crustaceous Le- 

 pralia to the plant-like Salicomaria; or, lastly, 

 from the locomotive Cristatella to the fixed confer- 

 void Plumatella. These remarkable little animals 

 are usually found adhering by a kind of root to 

 stones, rocks, and other marine bodies, their delicate 

 and fragile forms being swayed to and fro by the 

 motion of the water that surrounds them ; and, 

 often after storms, they strew the sandy shores with 

 their elegant uprooted fronds. The increase of the 

 Polyzoa is by buds or gemmules, but some among 

 them are developed from ova, which minute eggs 

 are covered by a remarkable shell, being furnished 

 with numbers of little horny hooks to enable them, 

 to retain their hold upon marine bodies. 



IV. CLASS.— ASCIMAN-POLYPS (Polyzoa). 



Animal polypiform, enclosed in horny or calca- 

 reous cells, united together in a common mass ; diges- 

 tive canal with a distinct mouth and vent ; mouth 

 surrounded with eight or more, ciliated, retractile 

 tentacles. 



