, 
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 cesophagus. 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 cesophagus is the large subceso- 
phageal ganglion which is connected with the brain by the two 
connectives already mentioned. ‘The brain or supracesophageal 
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. 
