A CRAYFISH OR A LOBSTER 41 



The nervous system consists of a ventral double nerve cord 

 lying in the mid- ventral line in the body-cavity and extending 

 the length of the animal, with paired ganglia at intervals, also 

 of a brain situated just back of the eyes, which is united with 

 the ventral nerve by two nerve connectives, passing one on each 

 side of the oesophagus. The ventral ganglia have typically a 

 metameric significance, but many of the somites have lost their 

 ganglia, 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 ganglia. How many do you 

 count? Notice the lateral nerve-branches. In the cephalo- 

 thorax 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 cephalo- 

 thorax and working forward. How many thoracic ganglia do 

 you find? Just back of the oesophagus is the large suboeso- 

 phageal 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 

 system, showing accurately the number of ganglia and the 

 segments in which they lie, together with the lateral nerves. 



Exercise 27. Remove the brain and draw an outline of it on 

 a scale of 6 or 8, using a dissecting microscope or hand 

 lens. Show the antennal and the optic nerves. 



Exercise 28. 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. 



