258 MANUAL OF ZOOLOGY. 



MOLL USCOIDA. 



CHAPTER XLIL 

 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 denned 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 sometimes foliaceous (fig. 95, 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 a Polyzob'n 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 



