I02 ARTHROPODS 



(lo) Note the shape and outer appearance of the stom- 

 ach. Probe through the mouth into the stomach, and note 

 the varying hardness of the different parts. Find the in- 

 sertion of the muscles of the stomach. Are there any 

 pouchlike extensions of the pyloric portions? The organ 

 is chitinoits in places and resembles the outer carapace. 

 Open the stomach with the scissors. Note the hardened 

 areas and make out the uses of the projections. Draw 

 interior view of the stomach. 



(ii) The nervous system being on the ventral side 

 has not yet been injured. It consists of a series of ganglia 

 connected by nerve cords. The following may be found: 



The supra-esophageal ganglion, composed of two ganglia 

 fused together, is under the rostrum behind the antennae. 

 From this mass the two cords, or commissures, pass around 

 the esophagus and unite to form the infra-csophageal gan- 

 glion. Behind these, the ganglia and commissures extend 

 to the posterior portion. Study them. Do the ganglia corre- 

 spond to the number of body segments ? Do you find the large 

 nerve cords by which the muscles and other parts are sup- 

 plied with nerves ? These are really bundles of nerves which 

 divide and sub-divide until the single nerve only remains to 

 act upon the tissue. Make enlarged drawing of the nervous 

 system. 



Development. — In the early spring the eggs of the cray- 

 fish are laid and carried on the swimmerets of the female. 

 When the eggs are hatched the young still cling to the 

 swimmerets until they are able to care for themselves. The 

 shell is molted several times during the growth of the 

 animal and occasionally after it has reached maturity. In 

 molting, the carapace splits and the animal extricates itself 



