MOLLUSCOIDA I POLYZOA. 299 



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- 

 longing specially to any individual. The chief bond of con- 

 nection would appear to reside partly in the continuity of the 

 external integument, and partly also, in all probability, in a 

 slow interchange of the vital fluid with which the cavities of 

 the cells are charged." 



Fig. 122. Flustra foliacea, one of the Sea-mats, a Portion of the colony, natural 

 size ; b A fragment magnified, to show the cells in which the separate polypides 

 are contained. 



In one sub-order of the Polyzoa (Ctenostomata,} the polyzo- 

 arium consists of a series of cells arising from a common tube, 

 but this exception does not affect the value of the above 

 general distinction between the Polyzoa and the Hydroida. 



A second point of difference is found in the invariably cor- 

 neous (or chitinous) texture of the polypidoms of the Hydroida, 

 whereas those of the Polyzoa may be corneous or fleshy, but 



