IT.] THE CRAYFISH AND LOBSTER. 229 



wards and downwards to reach the base of the 

 antenna. 



3. Examine the circumoesophageal commissures under a 

 lens ; they will be seen to be connected by — 



a. The postasophageal commissure; a delicate trans- 

 verse band, slightly in front of the suboesophageal 

 ganglion. 



b. The antcesophageal commissure ; a similar structure 

 to «, arising at the sides and passing round the front 

 of the gullet. 



c. The 7nedian-ventral commissure; a very delicate 

 trunk, passing back from the supraoesophageal 

 ganglia in the middle line, to meet b. 



d. The anterior visceral nerve ; a median nerve arising 

 from the point of union between b and c ; it passes 

 upwards and forwards to reach the roof of the 

 stomach, upon which it subdivides for distribution 

 to its walls. 



e. The posterior visceral nerves — arising, one on either 

 side, from the last abdominal ganglia. They pass 

 slightly backwards to reach the base of the intes- 

 tine, the side walls of which they skirt. 



4. A companion dissection may profitably be made, by 

 bisecting the whole body a little to one side of the 

 middle line. 



The real double nature of the entire nervous axis 

 may best be appreciated by removing it and examin- 

 ing it on a blackened surface, under water. 



5. a. Te^se out a bit of perfectly fresh nerve- cord in 



water and stain with magenta or eosin. 



