ix MOLLUSC A 315 



ectodermal invagiuations. They are in reality, however, only deeper 

 portions of the mantle groove which intervenes between the mantle 

 fold and the body wall (Fig.' 249). 



The embryonic stomach, whose cells are gorged with albuminous 

 matter, has been shifted into the visceral hump, and in this way the 

 alimentary canal takes on a U shape. The intestine is lined by small 

 cubical cells, and, by an extension of cells of this description along 

 the mid-dorsal and mid-ventral lines to meet the stoinodeal cells, the 

 embryonic stomach becomes divided into two lobes which, in later 

 life, become converted into the two lobes of the liver. The median 

 portion lined by small cells forms the adult stomach, and the radula 

 sac arises as a ventral pocket of the stomodaeuni. 



Meanwhile the rudiments of the sense organs appear. The 

 otocysts (ot, Fig. 250) arise as pocket-shaped imaginations of the 

 ectoderm at the side of the foot. The eyes arise as similar invagina- 

 tions of the pretrochal area (oc, Fig. 250) ; they are formed at the 

 bases of two conical projections which are rudiments of the tentacles 

 (ten, Fig. 250). 



The principal ganglia of the central nervous system arise as 

 separate ectodermic thickenings, the commissures connecting them 

 being formed only afterwards. The cerebral ganglia arise later as 

 two thickenings of the velar region close to the eyes. The pedal 

 ganglia arise similarly from the post-velar region close to the 

 otocysts, at the sides of the foot. The pleural ganglia arise on the 

 sides of the body higher up and further back ; and lastly the visceral 

 ganglia arise from the ectoderm of the mantle cavity, that is, the 

 deepest and most posterior part of the mantle groove. All these 

 ganglia and their commissures are established before torsion begins. 



Torsion now takes place, and the mantle cavity, with the opening 

 of the anus and the two visceral ganglia, is rotated upwards and to 

 the right, so that it passes along the right side in an oblique line 

 until it reaches its permanent position on the back of the neck. 

 This torsion involves the lengthening of the intestine into a 

 recurrent loop, and the passage of the original right visceral ganglion 

 upwards and to the left, where it forms the supra-intestinal 

 ganglion, whilst the original left one is displaced to the right side 

 and forms the sub-intestinal ganglion. The right ureter passes 

 upwards and to the left, and the left one eventually takes up a 

 position on the right below it (Fig. 250, B). The pericardium and 

 the persistent right kidney are displaced from their original position, 

 underneath the gut, to a lateral position in which the kidney is 

 above the pericardium. 



When the torsion is complete the velar cells disappear and the 

 tentacles become long, while the foot develops its crawling surface, 

 and on the upper aspect of its posterior portion the operculum 

 appears. The gill appears as a series of outgrowths from the roof 

 of the mantle cavity, and the embryo then takes on the general 

 appearance of the adult (Fig. 251). 



