140 GASTROPODA. 



described through the rising up and growth of the margin of the 

 mantle, but it also becomes affected by the asymmetrical develop- 

 ment of the embryo. It is soon evident that the part of the cavity 

 lying on the right side is much deeper than that lying on the 

 left, and the cavity shortly becomes confined almost entirely to the 

 right side in consequence of the twisting of the embryo. On 

 this side there opens into it not only the rectum, but the efferent 

 ducts of the now developed definitive kidney and of the genital 

 organs. At a later stage, the mantle-cavity extends dorsally and 

 thence over to the left side. Above the mantle lies the shell which 

 passes from its earlier flat shape to a more arched form, till it becomes 

 a somewhat deep cup (Fig. 59 A and B), and, finally, in consequence 

 of its one-sided growth, becomes coiled (cf. p. 147). 



While the above processes have been gbing on, the anterior part of 

 the body also has undergone essential alteration. The velum has 

 degenerated more and more, while the foot has greatly increased in 

 size (Fig. 59 A-C). At its base, the otocysts (ot) have appeared as 

 ectodermal depressions which soon became cut off' as closed vesicles. 

 In the posterior dorsal part of the foot, the operculum is secreted in 

 a manner similar to the secretion of the shell (Fig. 99, o/i, and Fig. 

 92, op). 



The tentacles arise on the velar area as two very large swellings 

 which soon increase in height and thus become conical (Fig. 59 A-C, 

 t). At their bases the eyes appear. At the stage depicted in Fig. 

 59, both these organs can be recognised as belonging to the 

 velar area, since the ciliated ring is still present as a narrow band. 

 In these later stages, when, in keeping with the shape of the body, 

 the velum has become almost bilobed, it may be compared with the 

 sail of the Veliger larva, of the marine Gastropods which, however, is 

 much more distinctly bilobed. 



The further development of the embryo is chiefly determined by 

 the continued growth of the visceral sac as a result of the perfecting 

 of the inner organs, and by the increase in size of the foot and of the 

 tentacles. This may best be seen by comparing Fig 59 with Figs. 99 

 and 100. 



Before closing this section we must deal with one or two other 

 morphological points which could not earlier receive the consideration 

 they deserve. The first of these concerns the shape and transforma- 

 tion of the blastopore. In its simplest form, the blastopore has been 

 described as a rounded aperture appearing at the vegetative pole ; 

 this aperture, without undergoing essential change of form, may pass, 



