MOLLUSCOIDA: POLYZOA. 259 



intercommunication, comparable in kind perhaps, though not 

 in degree, to what obtains in many of the compound Ascidi- 

 ans. That this community exists is proved by the otherwise 

 inexplicable 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." 



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 

 are in the majority of instances more or less highly charged with 

 carbonate of lime. 



The homomorphism, however, which subsists between the 

 Polyzoa and the Hydroida is shown most decisively not to be 

 a true affinity, when the structure of the individual zooids is 

 examined. The polypite of a Hydroid Zoophyte, as we have 

 already seen, possesses no alimentary canal distinct from the 

 general cavity of the body; there are no traces of a nervous 

 system, and the reproductive organs are in the form of exter- 

 nal processes of the body-wall. In the zooid of all the Polyzoa 

 (fig. 94, 2), on the other hand, there is a distinct alimentary 

 canal, completely shut off from the somatic cavity; a nervous 

 system is present, and the reproductive organs are contained 

 within the body. 



The following are the more important differences in the ter- 

 minology employed to designate the various parts of the com- 

 pound growths of the Polyzoa and the Hydrozoa. In the 

 Hydroida the entire colony is called the " hydrosoma," and its 

 investing layer, when present, is called the "polypary," or 

 " polypidom," whilst the individuals composing the hydrosoma 

 are called the " polypites," and the cups in which these are 

 in some cases contained are called " hydro thecae." In the 

 Polyzoa the entire colony or its entire dermal system is 

 called the " polyzoarium," or " ccencecium ; " the separate 

 zooids are called " polypides," and the little chambers in which 

 each is contained are called the " cells." 



It will be seen, therefore, that the term polypite is restricted 

 to the zooid of a compound Hydrozoon, or to the entire hydro- 



