A LOBSTER OR A CRAYFISH. 59 



n) The nervous system. This consists of a supra-cesophageal ganglion 

 or brain located just back of the eyes. It sends off several 

 cerebral nerves which may be noted. It also gives rise to a pair 

 of commissures which pass around the oesophagus and connect 

 it with another ganglion, the sub-oesophageal. From the latter a 

 double ventral nerve cord, with paired ganglia at intervals and 

 with numerous branch nerves, runs the length of the body. In its 

 passage through the cephalothoracic region it follows the sternal 

 channel, formed of transverse ridges of the hard ventral wall. 

 Break away this structure so that the cord may be exposed for its 

 entire length. The double nature of the cord is apparent at the 

 point where the sternal artery passes through it. 



Exercise 6. Make a diagram of the nervous system on the same scale 



as in Exercise i. 

 Exercise 7. Make separate sketches of the heart and the gastric mill 



of the stomach. 

 Exercise 8. Separate a ring of the abdomen with its attached swim- 



merets, remove the muscle from the inside of the ring, and draw on a 



slightly enlarged scale. 



