NERVOUS SYSTEM OF CRUSTACEA. 



543 



here perfectly distinct from one another, and 

 even separated by a small interval : they present 

 a series of ganglia, which are nearly of equal 

 size, and equidistant from one another ; one pair 

 corresponding to each segment of the body,* 

 and vmited by transverse threads ; and other 

 filaments, diverging laterally, proceed from each 

 ganglion. During the progress of growth, the 

 longitudinal cords approach somewhat nearer to 

 each other, but still remain perfectly distinct. 



439 



440 



The first pair of ganglia, or the cephalic, have 

 been considered, though improperly, as the brain 

 of the animal. 



The next step in the gradation occurs in the 



* These segments are numbered in this and the following 

 figure in their proper order, beginning with that near the head. 

 A is the external antenna ; a, the internal antenna ; and e, the 

 eye. 



