154 



GASTROPODA. 



observed by Butschli, the plug was said to be expelled before the 

 actual shell formed, whereas Fol believes here that this mass which 

 fills the shell-gland passes directly into the shell when that depression 

 flattens out again. 



In the stage depicted in Fig. 64, the pre-oral ciliated ring has 

 made its appearance and in this way the velar area (vv) becomes 

 bounded. Behind the mouth, the rudiment of the foot (p) appears 

 as a prominence which widens and thus assumes the form of a plate 

 (Fig. 65 B). On either side at its base, the otocysts (ot) appear, 

 while, anteriorly, the bilobed pedal gland forms as an ectodermal 

 invagination. The posterior part of the foot at this early stage 

 secretes a thin plate (op) which, in position and function, coi-responds 



a. 



eJ3. 



Fig. 65. — Embryos of Firoloida Desmaresti. A, seen from the right side, B, from 

 the ventral side (alter Fol). ac, anal cells ; d, posterior part of the enteron ; /, foot \. 

 fd, pedal gland ; m, mouth ; md, enteron ; op, operculum ; ot, otocyst ; .?, shell ; 

 sp, apical plate ; w, ciliated ring. 



to the operculum of the Prosobranchia. Fine calcareous concretions 

 become deposited beneath the shell-integument, and lead to the 

 development of the calcareous shell. Unequal growth here also 

 causes the shell soon to assume a coiled form, at least in the later 

 stages. In Firoloida and Pterotracliea, the shell has only two 

 whorls ; in Garinaria and Atlanta it coils several times. 



Up to this [joint, the alimentary canal is without an anus. 

 According to Fol, two large cells which appear behind the foot 

 indicate, even in the stage depicted in Fig. 64, the position of this 

 organ, and at this point the enteron, which is bent anteriorly, be- 

 comes connected with the somewhat depressed ectoderm (Fig. 65, ac). 



