COMPARISON WITH POLYZOA. 31 



arranged round the mouth. On examination 

 under the microscope, however, a considerable 

 advance upon the structure of the hydroid will 

 be observed. There is a well-formed through 

 alimentary canal, entirely cut off from the body 

 cavity, there is also a gizzard, and well-defined, 

 though simple, muscular and nervous systems. 

 The polypary, as it may be called, partakes to 

 some extent, of a horny substance, but 

 carbonate of lime also enters largely into the 

 composition, more especially in the higher 

 forms. The receptacle also, or cell of the 

 polyzoan is more a part of the animal than 

 is the case in the hydroids, where the whole 

 polypary hangs like a loose-fitting garment 

 on the compound animal, the former condition 

 resembling more the relation of the shell to 

 the shell-fish in the ordinary mollusc, to which 

 the polyzoan is nearly allied. 



