238 MANUAL OF ZOOLOGY. 



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, 

 which retract the upper portion of the polypide within the 

 cell. These muscles arise from the inner surface of the endo- 

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

 upper part of the oesophagus. The polypide, when retracted, 

 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. 72, 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. The products of generation, i. e. the spermatozoa 

 and ova, are discharged into the perigastric space, where 

 fecundation takes place ; but it is not certainly known how 

 the impregnated ova escape into the external medium. 



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 developed in the interior of the polypide. 

 The statoblasts are found, in certain seasons, lying loose in the 

 perigastric cavity. In form ' they may be generally described 

 as lenticular bodies, varying, according to the species, from an 

 orbicular to an elongated-oval figure, and enclosed in a horny 

 shell, which consists of two concavo-convex discs united by 

 their margins, where they are further strengthened by a ring 

 which runs round the entire margin, and is of different struc- 

 ture from the discs When the statoblasts are placed 



under circumstances favouring their development, they open 

 by the separation from one another of the two faces, and 

 there then escapes from them a young Polyzoon, already in an 

 advanced stage of development, and in all essential respects 

 resembling the adult individual in whose cell the statoblasts 

 were produced.' (Allman.) The statoblasts are formed as 

 buds upon the ' funiculus ' the cord already alluded to as 



