THE DEVELOPMENT OF THE ODOXTOrHORA. 429 



but the intestine passes into the large visceral sac instead of 

 into the cavity of the mesosoma. The shell is stated by 

 Gegenbaur to be at first internal, as in Limax. In neither 

 case has the relation of the shell to the shell-gland been 

 determined. 



The process of development appears to present a consider- 

 able range of variation in the I^ulnionata. Semper* states 

 of a species of Vaginulus, that, after the process of cleavage, 

 the embryo assumes the form of a cylinder, at one pole of 

 which the rudiments of the tentacula and of the lips appear ; 

 while, at the sides, a longitudinal ridge indicates the edge of 

 the mantle, and marks off the more convex pallial region 

 from the flat foot. No shell is formed. 



In LymncBus^ as has been already stated, the vitellus 

 undergoes complete division, and the resulting vesicular 

 morula undergoes invagination to produce the hypoblast. 

 Only the middle part of the archenteron becomes the alimen- 

 tary canal, however. The lateral portions, which take on the 

 form of rounded sacs, may not improbably, as in the Bracliio- 

 pods, give rise to the perivisceral cavity, though this has not 

 been proved. The mouth is produced by the formation of an 

 opening in the coalesced endoderm and ectoderm, at a point 

 near the anterior end of tlie body. Upon each side of the 

 mouth a transverse ciliated ridge of the ectoderm is developed, 

 and represents the edge of the velum in other molluscan em- 

 bryos. Behind this, and on the opposite side of the embryo 

 to that on which the mouth is placed, a raised patch of the 

 ectoderm represents the mantle. The foot commences as a 

 papilla immediately behind the mouth. An involution of the 

 centre of the pallial ectoderm gives rise to a shell-gland, but 

 the proper shell is developed, independently of this, as a cu- 

 ticular secretion from the whole surface of the mantle. 



Thus the embryo of JLymnceus possesses an incompletely 

 developed velum, and is, in all essential respects, similar to 

 the veligerous embryo of Lamellibranchs, Pteropods, and 

 Gasteropods ; while the Slugs and Land-snails have neither 

 the velum (unless it be represented by the anterior contrac- 

 tile sac) nor the external embryonic shell. 



The development of the Cephalopoda is very unlike that 

 of other Mollusks, and will be dealt with under the head of 

 that group. 



» " EntwickelunffsgeseWehte der AmpuUaria polita.'''' 



2 Lankester, " Observations on the Development of the Pond-Snail, Lym- 

 n(tvs stagnalis.''^ (^Quarterly Journal of Microscopical Science^ vol. xiv., New 

 Series.) 



