MOLLUSCOIDA I POLYZOA. 305 



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 

 cells of the marginal ring, or " annulus," being spongy and 

 filled with air. They must be looked upon as "gemma pecu- 

 liarly encysted, and destined to remain for a period in a qui- 

 escent or pupa-like state " (Allman). 



As regards the development of the Polyzoa, the embryo 

 upon its emergence from the ovum presents itself as a ciliated, 

 free-swimming, sac-like body, from which the polypide is sub- 

 sequently produced by a process of gemmation. In the sin- 

 gular Rhabdopleura the primitive bud is enclosed between two 

 fleshy lobes or valve-like plates, attached along their dorsal 

 margin, and giving exit in front to the rudimentary lophophore. 

 As the development proceeds, these plates cease to keep pace 

 in their growth with the rest of the bud ; till ultimately they 

 appear as a peculiar shield-like organ on the haemal side of the 

 lophophore. These lobes have been compared by Dr Allman 

 with the mantle-lobes of the Lamellibranchiata. 



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