296 ELEMENTARY BIOLOGY. [CHAP. 



The walls of the vessels concerned in respiration 

 are frequently pigmented. The entire pulmonary 

 system can therefore be generally followed in an 

 uninjured specimen, without the aid of injection, by 

 examining with care the wall of the lung-sac from 

 the outside. 



I. The nervous system. 



1. Pin the animal down right side uppermost and open 

 up the lung-sac, cutting along the edge of the rectum; 

 divide the latter, together with the adjacent excretory- 

 duct and lung-sac, across its middle transversely, and 

 reflect the left hand half. Next remove the right 

 half of the wall of the body and visceral-sac, together 

 with the tentacles of the same side and the greater 

 portion of the genital apparatus. Remove the foot 

 to the level of the pedal gland, and with it the right 

 half of the retractor pedis muscle. Follow in the 

 order given : 



a. The central nervous system. It consists of a cir- 

 cum-oesophageal mass, buried up in a loose con- 

 nective-tissue sheath. Carefully dissect away the 

 superficial portion of the sheath, until the nerve' 

 ganglia are exposed (looked at thus from the side, 

 but one set will be seen) ; they are — 



b. The supra- oesophageal ganglia ; two considerable 

 masses lying above the gullet, connected together 

 by a short band-like commissure. 



c. The pedal ganglia ; two large masses lying im- 

 mediately above the pedal-disc. They distribute a 

 considerable series of fibres ventrally to the foot 

 and pedal gland, and a smaller set laterally to the 

 adjacent body-wall. Each pedal ganglion is united 



