372 



MANUAL OF ZOOLOGY. 



but this hardly affects the general value of the distinction 

 above spoken of. 



The homomorphism, however, which subsists between the 



Fig. 197. 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. 



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. 198, 3), 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. 



In the Polyzoa, the entire colony or its entire dermal system 

 is called the " polyzoarium " or " coenoecium ; " the separate 



