180 GASTROPODA. 



which has now become cap-like. The margin of the shell seems 

 buried in a groove, a swelling of the ectoderm, the margin of the 

 mantle, having formed here. The whole embryo has somewhat 

 lengthened, and the foot stands out more distinctly (Fig. 79). 



The foot in Limnaea, which at first appears as an unpaired swelling, is 

 said to assume a bilobed form (Ray-Lankester). Such a bilobed foot seems 

 often to occur among the Gastropoda. We have already met with it in 

 Succinea, Patella and Vermetus (p. 132). Fol also observed this later develop- 

 ment of the bilobed form in the foot of Limnaea, as well as in Planorbis and 



ma. 



ere, 

 i 



a* / . 





au. 

 / 





y-J' 



ot> 







';• 



/ 



Pig. 79. — Older embryo of Planorbis, seen from the side (after Rabl). au, eye;/, 

 foot ; ma, margin of the mantle ; md, enteron and digestive gland (large cells) ; 

 at, otocyst ; pg, pedal ganglion ; r, radular sac ; s, shell ; t, tentacle ; un, primitive 



Ancylus, though in these last two animals it was less striking. Ray Lan- 

 kester compares this to the transformation of the foot into the paired fin in 

 the Pteropoda. 



The outgrowth of the body-epithelium to form the foot causes a 

 considerable enlargement of the ventral portion of the inner cavity 

 of the larva, and a similar cavity is produced pre-orally by the dilata- 

 tion of the part which is encircled by the velum. A similar process 

 has already been met with in the Prosobranchia (p. 150). The anterior 

 swollen part of the embryo is known as the cephalic vesicle and the 



