MOLLUSCA: POLYZOA. 377 



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. 198, 2), placed upon one side of the oeso- 

 phagus, between it and the anal aperture, and apparently 

 really of a double nature. Besides the single ganglion which 

 belongs to each polypide, there is also in some 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 continue their movements and 

 retain their irritability after the death of the polypides ; but 

 high authorities deny that the so-called "colonial nerve-system" 

 is really of a nervous nature at all. 



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. 198, 2, g), 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. 198, 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 

 fecundation takes place ; and the impregnated ova escape by 

 special openings in the body-wall, by dehiscence of the cell, 

 or in some manner not as yet thoroughly understood. 



As already mentioned, continuous gemmation occurs in all 

 the Polyzoa, the fresh zooids thus produced remaining attached 

 to the organism from which they were budded forth, and thus 

 giving rise to a compound growth. 



A form of discontinuous gemmation, however, occurs in many of the 

 Polyzoa, in which certain singular bodies, called " statoblasts, " are devel- 



