MOLLUSCOIDA : POLTZOA. 237 



tively termed the c Hippocrepian ' Polyzoa. In all the Polyzoa 

 in which this crescentic condition of the lophophore exists, 

 there is also a singular valve-like organ which arches over the 

 mouth, and is termed the ' epistome.' 



Fig. 73. Plumatella repens, one of the Fresh-water Polyzoa, enlarged. 

 (AfterAllman.) 



The mouth conducts by an oesophagus into a dilated stomach. 

 In some cases a pharynx may be present, and in others there 

 is in front of the stomach a muscular proventriculus, or giz- 

 zard. From the stomach proceeds the intestine, which shortly 

 turns forwards to open by a distinct anus close to the mouth. 

 As the nervous ganglion is situated on that side of the mouth, 

 towards which the intestine turns in order to reach its ter- 

 mination, the intestine is said to have a ' neural flexure,' and 

 this relation is constant throughout the entire class. 



Respiration in the Polyzoa appears to be carried on by the 

 ciliated tentacles, and by the * perigastric space,' which is filled 

 with a clear fluid, containing solid particles in suspension. A 

 kind of circulation is kept up in this ' perigastric fluid,' by 

 means of the cilia lining the inner surface of the endocyst. 

 Beyond this there is nothing that could be called a circulation, 

 and there are no distinct circulatory organs of any kind. 



The nervous system in all the Polyzoa consists of a single 

 small ganglion (fig. 72, 2), placed upon one side of the oeso- 

 phagus, between it and the anal aperture. Besides the single 

 ganglion which belongs to each polypide, there is also in many, 

 if not in all of the Polyzoa, a ' colonial nervous system.' That 

 is to say, there is a well developed nervous system, which 

 unites together the various zooids composing the colony, and 



