DEVELOPMENT OF THE EXTERNAL FORM — PROSOBRANcrflA. 151 



HU. 



•a.. 



enteron is seen to be open towards the yolk (Figs. 47 C and I), 62 B, 

 and 63), which occupies the posterior and dorsal portion of the em- 

 bryo. The enteron consists of an anterior wider section and a posterior 

 tubular section (Figs. 47 I), and 62, mil). The latter is at first 

 parallel to the longitudinal axis, but soon lies obliquely to it, becomes 

 connected with the ectoderm, and opens out through the anus which 

 still lies in the ventral middle line (Fig. 61 C). At a later stage, 

 the posterior part of the 



intestine assumes a still ££, 



more oblique position 

 and the anus comes to 

 lie on the right side 

 (Fig. 61 I) and E). 

 Here also the pallial 

 cavity arises as a sickle- 

 shaped depression of the 

 ectoderm, this cavity in 

 Nassa being altogether 

 restricted to the right 

 side of the embryo. The 

 asymmetry seems still 

 more marked here than 

 in Palndina (p. 138). 

 The shell also shares 

 in this asymmetry ; by 

 its rapid growth it has 

 become cup-shaped and 

 covers the greater part 

 of the visceral dome 

 (Fig. 61 D). The 

 operculum appears as a 

 delicate plate in the 

 posterior dorsal part of 

 the foot (C and D, op). 



In the foot can be seen a ventral tubular ectodermal depression, 

 which is no doubt the rudiment of the pedal gland (Figs. 61 E 

 and I), and 63). The velum, which is not yet closed dorsally, 

 has lost its former almost circular shape through the shifting 

 forward of the mouth and the appearance at this point of a notch 

 (Fig. 61 C). It at the same time increases in size and thus assumes 

 the bilobed form which we have already described in connection with 



vtL— 



Fr;. 62. — .1. surface view, ami B, median longitudinal 

 section through an embryo of Fus-us (after Bo- 

 hketzky). d, yolk ; /, foot : M), cephalic vesicle; 

 Oliver; m, mouth; md, enteron; mg, stomach; 

 <>t, otoeyst ; s, shell; ', tentacle: v, velum : vd, 

 stomodaeum ; :., sub-velar cells. 



