MOLL USCO I D A. 



CHAPTER XLI. 



POLYZOA. ft s^yY" 



DIVISION A. MOLLUSCOIDA. Nervous system consisting of a 

 single ganglion, or of a principal pair with accessory ganglia ; no 

 distinct organ of the circulation, or an imperfect heart. 



This division includes three classes viz., the Polyzoa, the 

 Tunicata, and the Brachiopoda, 



CLASS I. POLYZOA (Bryozoa). The members of this class 

 are denned as follows: "Alimentary canal suspended in a 

 double-walled sac, from which it may be partially protruded by a 

 process of ei>agmation, and into which it may be again retracted 

 by invagination. Mouth surrounded by a circle or crescent of 

 hollow, ciliated tentacles ; animals always forming composite 

 colonies " (Allman). 



All the Polyzoa live in an associated fbnnjn colonies or 

 "polyzoaria," which are sometimes foliaceous (fig. 197), some- 

 times branched and plant-like, sometimes encrusting, and very 

 rarely are- free. Each "polyzoarium" consists of an assem- 

 blage of distinct but similar zooids arising by continuous gem- 

 mation from a single primordial individual. The colonies thus 

 produced are in very many respects closely similar to those of 

 many of the Hydroid IJolypes, with which, indeed, the Polyzoa 

 were for a long time classed. The " polyzoarium," however, 

 of a 'Polyzoon differs from the polypidom of a composite Hy- 

 droid in the general fact that the separate cells of the former 

 do not communicate with one another otherwise than by the 

 continuity of the external integument; whereas the zooids of 

 the latter are united by an organic connecting medium, or 

 " ccenosarc," from which they take their origin. 



In one group of the Polyzoa viz., the Ctenostomata (includ- 

 ing Vesicularia and its allies), the cells arise from a common 

 stalk, and are thus placed in communication with one another; 



