— 42 — 



point. Cut the tracheae of one side extending to the remain- 

 ing part of the alimentary canal, so that it can be pushed 

 away to the other side. Be careful not to injure this part 

 of the alimentary canal till after the vagus nerve has been 

 studied, as indicated in a later paragraph. Take the speci- 

 men out of the water, and put a few drops of carmine solu- 

 tion on the muscles and nerves in the abdomen. Return 

 the specimen to the dissecting dish and try to observe the 

 peritoneum. This is a transparent, apparently structure- 

 less membrane, stretched over the floor of the abdominal 

 cavity in such a way as to protect the central part of the 

 nervous system. Owing to the transparency of this mem- 

 brane, it is very difficult to see it ; but when a specimen 

 is treated as indicated above, the fine particles of carmine 

 that become lodged beneath the peritoneum render it visi- 

 ble, especially when the carmine is washed off from the 

 muscles that are not covered by the membrane. The peri- 

 toneum is attached along each side of the body just lat- 

 erad of the great ventral muscles ; the points of attachment 

 are on the lines separating the segments of the body. Be- 

 tween the points of attachment, the margins of the mem- 

 brane curve mesad, giving the membrane the appearance of 

 being strongly stretched at the points of attachment. 



The Nervous System. — After removing the alimentary 

 canal from the specimen opened along the dorsimeson, the 

 central nervous system will be exposed. 



Make a diagram showing the disposition of the ganglia 

 and of the principal nerves of the thorax and abdomen. 



Make careful dissections of the ganglia and nerves found 

 in the head, and make diagrams showing their arrangement. 



The following parts should be observed and figured : — 



The supraccsophageal ganglia. — These are two, large, 

 ovoid ganglia, lying above the oesophagus, and connected 

 by a short, thick commisure. 



