28 BRITISH HYDROID ZOOPHYTES. 



of the horny covering, and also of the 

 polypites, leaving only the stem and branches, 

 i.e., a system of tubes or channels (coenosarc), 

 with their ends open. Let there now be 

 developed between the inner and outer layer 

 (of which the coenosarc, like the polypite, is 

 composed) a middle and a much thicker layer 

 of simple flesh substance called protoplasm, 

 containing, however, numerous " flesh particles" 

 or cells. If we then picture the cilia through- 

 out this canal system maintaining by their 

 movements a constant circulation, not of partly 

 assimilated nutriment as in the hydroid colony, 

 but water containing food which is supplied 

 it to all parts of the system ; and furthermore, 

 that the larger end of the main channel or 

 central stem, into which all other channels 

 eventually lead, be open forming an exit for 

 the impoverished water, we shall gain an 

 approximate idea of the comparative systems 

 of a hydroid colony and a simple sponge. 



As we go higher in the sponges, instead 

 of the channel being lined with cilia 

 and exercising stomachic functions, the cilia 

 become restricted to little chambers lined by 

 cells of peculiar form (collar cells), where also 



