• APLYSTA. 231 



and the nervous tissue lies a blood sinus. In the anterior 



regions of the body it is sometimes difficult to distinguish 



between nerves and the delicate muscle tendons that hold the 



gut in place. The presence of the blood sinus surrounding 



the nerve, however, at once distinguishes the latter. 



[Dissection. Make a median incision through the body 

 wall between the rhinophores so as to expose the buccal bulb. 

 Extend the cut forwards and backwards until the cerebral 

 ganglia are visible. Find the pleural ganglia and trace the 

 pleuro-visceral cords back to the adjacent parietal and visceral 

 ganglia, which are situated dorsal to the alimentary canal 

 and at the level of the posterior curve of the crop, exactly 

 beneath the anterior wall of the pericardium. This involves 

 making an oblique incision towards the genital aperture. 

 Continue the cut through the floor of the pallial cavity between 

 it and the base of the right parapodium.] 



A. Cerebral Ganglia and Nerves. (Fig. 16.) 



The cerebral ganglia (c. g.) lie on the dorsal side of the 

 oesophagus at its point of origin from the buccal mass. So 

 large are the nerve cells in these and the other chief ganglia 

 that the surface possesses a granulated appearance. The 

 antero-lateral surface of each cerebral ganglion is somewhat 

 swollen, and is seen in sections to consist of a distinct ganglion, 

 the optic ganglion of Mazzarelli, which is markedly dispro- 

 portionate to the minute eye of the animal. The two cerebral 

 ganglia are united by a single cerebral commissure, and are 

 linked up with the pleural, pedal, and stomato-gastric or 

 buccal ganglia by cerebro-pleural, cerebro-pedal, and cerebro- 

 buccal connectives respectively. 



From the cerebral ganglia arise the following pairs of 

 nerves, the distribution of right and left sides being similar 

 except in the case of 5 : — 



1. A small tegumentary nerve (teg. n. 1). This arises 

 from the antero-ventral portion of the ganglion and runs 

 forward to the skin between the tentacles. A branch of the 

 nerve innervates the buccal orifice, and the dorsal side and 



