MOLL U SCO ID A. 



CHAPTER XLII. 



POLYZOA. 



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 (Bryozod). — The members of this class 

 are defined as follows : — " Alimentary canal suspended in a 

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

 by a process of evagination, 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 form in colonies or 

 "polyzoaria," which are sometirhesfoliaceous (fig. 102, i), 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 Polypes, with which, indeed, the Polyzoa 

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

 of z. Polyzobn 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. On this 

 point Mr Busk observes : — 



" It has been before said that the Polyzoa are always asso- 

 ciated into compound growths, made up of a congeries of in- 

 dividuals, which, though distinct, yet retain some degree of 



