150 



GASTROPODA. 



becomes marked off from the principal part of the embryo which 

 contains the yolk (Fig. 61 D), the anterior part becoming swollen up 

 like a vesicle (Fig. 63, ce, v). This phenomenon can be observed, still 

 better than in Nassa, in a species of Fusu* examined by BOBRETZKY. 

 In this form, the foot and especially the anterior part of the body 

 appear to be swollen into a large vesicle (Fig. 62 A and B, kb], and 

 this part is therefore here also sharply marked off from the posterior 



7T 35 f* 



s. 



FIG. 61. A-JS, embryos of Nassa mutabilis of different ages (after BOBRETZKY). bl, 

 blastopore ; d, posterior tubular portion of the enteron ; dr, yolk ; ex, group of 

 ectodermal excretory cells ; /, foot ; fd, pedal gland ; h, rudiment of the heart ; id, 

 posterior hepatic lobe, near which can be seen, to the left, the anterior hepatic lobe, 

 and above the latter the intestine (d) and the anus ; k, rudiment of gill ; kh, pallial 

 cavity ; Ih, larval heart ; op, operculum ; r, margin of the shell (s) ; v, velum. 



part of the embryo. This swollen part, which corresponds to the 

 pre-oral section of the Trocliophore larva, and which is found in other 

 Prosobranchs, as well as in various other Gastropods (Pulmonates), 

 has been called the cephalic vesicle. The embryo in consequence 

 presents a very characteristic appearance (Figs. 62, kl, and 81, kbl). 



The condition of the entoderm or yolk is of special significance for 

 the embryos now under consideration. The sac-like rudiment of the 



