THE ONTOGENY OF THE STYLOMMATOPHORA. 



185 



the foot now bulges out and also commences to swell up into a vesicle. 

 Little now remains of the Trochophore shape. At a stage somewhat 

 younger than that depicted in Fig. 81, a slight vestige of the velum 

 is still to be found in two transverse ciliated ridges which lie on 

 either side of the mouth and run towards the shell-gland. These, 

 however, do not extend up to the mouth, and soon disappear. In 

 Arion and Limax, no traces of the velum are to be found (FoL). 

 These embryos, like those of the aquatic Pulmonates, are able to 

 rotate within the egg, being covered with cilia. 



s ?^2&' / ,--'" 



--fid. 



VIG. 83. Older embryo of Limax maximiis, seen from the side (after FOL). au, eye ; 

 <cg, cerebral ganglion; d, yolk-material;/, foot; It, labial palp ; ma, mantle-fold; 

 md, enteron and digestive gland ; ol, upper lip ; pd, podocyst ; py, pedal ganglion ; 

 rs, radular. sac ; s, shell ; t, tentacle ; un, primitive kidney. 



The position of the different organs of the embryo can be under- 

 stood most easily by reference to Fig. 81. The oesophagus is followed 

 by the enteron from which the digestive gland composed of large 

 albumeniferous cells is already becoming differentiated, posteriorly 

 the enteron is lined by smaller entoderm-cells. The anus lies behind 

 the pedal swelling, and behind it again, marking the dorsal side, is 

 found the shell-gland. A pit lying near the mouth represents the 

 rudiment of the radular sac which, according to FOL, arises in the 

 stomodaeum, which has not yet fully sunk in, and is thus near the 

 oral aperture, but is soon drawn into the buccal cavity. Near the 



