MOLLUSCOIDA : POLYZOA. 285 



(fig. 10 1, 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. loi, 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; 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 

 extending from the testis to the stomach — upon which they 

 may usually be seen in different stages of growth. They do 

 not appear to be set free from the perigastric space prior to 

 the death of the adult, and when liberated they are enabled 

 to float near the surface of the water, in consequence of the 



