MUSEUM OF COMPARATIVE ZOOLOGY. 



191 



the larger portion of the ganglion ; they are thicker and shorter than 

 the cerebro-pedal connectives, from which they are separated by only a 

 narrow space. 



The cerebral commissure is much shorter than before (Plate X. 

 Fig. 126), but it has not increased much in thickness (Plate VIII. 

 Fig. 101). In sagittal sections it is seen to be composed of a central 

 portion made up of nerve fibres cut crosswise and a peripheral layer of 

 nuclei ; but the nuclei are wanting on the face of the commissure which 

 is in contact with the dorsal wall of the oesophagus. 



Figure E. — The left surface of a section parallel to the sagittal plane from an embryo of the 



fourteenth day . X 73 



art. pd. Pedal artery. plu -pd. 



ceb. dx. Right cerebral ganglion. plu.-vsc 



ceb.-pd. Cerebro-pedal connective. pd. dx. 



cnch. Shell gland. rt. 



lab. Lip. ta. 



n. Nerve ta'. 



o cy. Otocyst. vsc. 



plu. Pleural ganglion. 



Pleuro-pedal connective. 

 Pleuro-visceral connective. 

 Right pedal ganglion. 

 Rectum. 

 Ocular tentacle. 

 Labial tentacle 

 Visceral ganglion. 



The pedal ganglia (Plate VIII. Figs. 97-100, Plate IX. Figs. 114, 

 118, 119) lie between the radula sac, which is above, and the foot gland 

 which is below them. They are nearer together than on the twelfth 

 day, and their anterior ends are more rounded (Fig. 114). Their pos- 



