414 MOLLUSCA. 



CLASS POLYZOA OR BRYOZOA. 



Animal composite, forming colonies, all the members of which 

 are produced ~by budding from a primitive being (zooid). Each 

 member of the colony (zooid) is enclosed in a double-walled sac, 

 the outer coat of which is mostly hardened by horny or calcareous 

 matter. There is no heart, and the mouth is surrounded by a 

 circle or crescent of hollow ciliated tentacles. The colonies are all 

 but invariably juiced to some foreign object, and are in many cases 

 plant-like inform. 



All the Polyzoa live in an associated form in colonies or 

 " polyzoaria," which are sometimes foliaceous, sometimes 

 branched (fig. 259) and plant-like, sometimes encrusting, 

 and very rarely are free. Each " polyzoarium " consists of 



Fig. 259. A, Fragment of an undescribed living species of Heteropora, from New Zealand, 

 of the natural size ; and B, Surface of the same, enlarged (original) ; c, Surface of a branch of 

 Heteropora subreticulata, from the Tertiary, enlarged (after Reuss). 



an assemblage of distinct but similar zooids arising by con- 

 tinuous gemmation from a single primordial individual. The 

 colonies thus produced are in very many respects closely 

 similar to those of many of the Hydroid Polypes, with which, 

 indeed, the Polyzoa were for a long time classed. The " poly- 

 zoarium," or " ccencecium," however, of a Polyzoon differs from 

 the polypidom of a composite Hydroid in the general fact 

 that the separate cells of the former do not communicate 

 with one another otherwise than by the continuity of the 



