MOLLUSCOIDA: POLYZOA. 263 



stage, which is termed by Professor Allman the " lophophore." 

 In the majority of Polyzoa including almost all the marine 

 species the lophophore is circular (fig. 95, 2), but in most of 

 the fresh-water forms it has its neural side extended into two 

 long arms, so that the entire lophophore becomes crescentic 

 or "horse-shoe-shaped" (fig. 95, 3); hence this section is 

 sometimes collectively termed the " 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." 

 The only marine forms in which the lophophore is bilateral 

 are Pedicellina and Rhabdopleura ; the only fresh-water species 

 in which the lophophore is orbicular are Paludicella and 

 Urnatetta. 



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 forward 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. 94, 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 

 brings them into relation with one another. It is probably in 

 virtue of this system that the avicularia are enabled to con- 

 tinue their movements, and retain their irritability after the 

 death of the polypides. 



The muscular system is well developed, and consists of 

 various muscular bands, with special functions attaching to 

 each. The most important fasciculi are the retractor muscles 

 (fig. 94, 2, g), which retract the upper portion of the polypide 



