304 MANUAL OF ZOOLOGY. 



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 termina- 

 tion, 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 suspen- 

 sion. 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. 123, 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. 123, 2, ), which retract the upper portion of the polypide 

 within the cell. These muscles arise from the inner surface of 

 the endocyst near the bottom of the cell, and are inserted into 

 the upper part of the oesophagus. The polypide, when re- 

 tracted, is again exserted, chiefly by the action of the "parietal 

 muscles," which are in the form of circular bundles running 

 transversely round the cell. 



As far as is known, all the Polyzoa are hermaphrodite, each 

 polypide containing an ovary and testis (fig. 123, 2). The 

 ovary is situated near the summit of the cell, and is attached 

 to the inner surface of the endocyst. The testis is situated at 

 the bottom of the cell, and a curious cylindrical appendage, 

 called the " funiculus," usually passes from it to the fundus of 

 the stomach. There are no efferent ducts to the reproductive 

 organs ; and the products of generation i.e., the spermatozoa 

 and ova are discharged into the perigastric space, where 



