232 MANUAL OF ZOOLOGY. 



MOLLUSCOIDA. 



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 (Bryozoa). 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 sometimes foliaceous (fig. 71), some- 

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

 rarely are free. Each ' polyzoarium ' consists of an assemblage 

 of distinct but similar zooids, arising by continuous gemma- 

 tion 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 Po- 

 lyzoa were for a long time classed. The ' polyzoarium,' how- 

 ever, 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 external integument ; whereas the zooids of 

 the latter are united by an organic connecting medium, or 

 * coenosarc,' 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 

 intercommunication, comparable in kind perhaps, though not 

 in degree, to what obtains in many of the compound Ascidians. 

 That this community exists is proved by the otherwise inex- 

 plicable circumstance that the polyzoaria in many instances 

 present elements common to the whole growth, and not be- 



