IT.] THE CRAYFISH AND LOBSTER. 22Q 



wards and downwards to reach the base of the 

 antenna. 



3. Examine the circumcesophageal commissures under a 

 lens ; they will be seen to be connected by 



a. The postazsophageal commissure; a delicate trans- 

 verse band, slightly in front of the suboesophageal 

 ganglion. 



b. The antozsophageal commissure; a similar structure 

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

 of the gullet. 



c. The median-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. Tease out a bit of perfectly fresh nerve-cord in 



water and stain with magenta or eosin. 



