2,2 PHYLUM ARTHROPOD A 



so that there are fewer ganglia than somites. The double nature 

 of the ventral nerve is best seen in the thorax. 



Remove all the muscles and the viscera from the body. The 

 ventral nerve cord will be seen in the abdomen lying in the mid- 

 ventral line. Notice the gangha. How many do you count? 

 Notice the lateral nerve branches. In the cephalothorax the nerve 

 cord is concealed beneath transverse ridges of the ventral wall of 

 the shell. Cut these with scissors and expose the nerve, beginning 

 at the hinder end of the cephalothorax and working forward. 

 How many thoracic ganglia do you find ? Just back of the oesoph- 

 agus is the large suboesophageal ganglion which is connected with 

 the brain by the two connectives already mentioned. The brain, 

 or supraoesophageal ganglion, is just back of the eyes. 



Exercise 26. Draw an outline of the body and in it the nervous sys- 

 tem, showing accurately the number of ganglia and the segments 

 in which they lie, together with the lateral nerves. 



Exercise 27. Draw a diagram representing an ideal sagittal section 

 of the animal in which the relative position of the principal systems 

 of organs is accurately shown. 



