1 84 ELEMENTARY BIOLOGY. [CHAP. 



The nervous system consists of a chain of thirteen gan- 

 glia — united by longitudinal commissures — lodged in the 

 median line of the ventral aspect of the body, from which 

 nerves are given to the organs of sense, to the muscles 

 of the trunk and limbs, and to the integument; and of a 

 visceral nervous system, developed chiefly upon the stomach 

 and hinder segment of the intestine. 



The ganglia are centres of aggregation of the nerve 

 cells ; of the thirteen seen in the adult the most an- 

 terior lies in the cephalon, close to the attachments of 

 the three anterior pair of appendages, and gives branches 

 to them and to the visceral nervous system. It is usually 

 termed the brain or the supracesophagcal ganglion. It is 

 connected by two commissural cords, which pass on each 

 side of the gullet, with a larger ganglionic mass, which is 

 called the subocsophageal ganglion. This occupies the 

 region of the hinder part of the cephalon and the anterior 

 part of the thorax, and gives off nerves to the mandible, 

 maxilloe and the three pairs of maxillipeds. Five other 

 ganglia lie in the five somites which bear the chelae and the 

 ambulatory limbs, and there is one for each abdominal 

 somite, the last of these being the largest of the six. 



The longitudinal commissures are double, and the 

 ganglia themselves shew more or less evident indications 

 of being double also. There is reason to believe that these 

 thirteen apparent ganglia really represent twenty pairs of 

 primitive ganglia, one pair for each somite ; the three an- 

 terior pairs having coalesced preorally to form the brain; 

 and the six which follow the mouth having united into the 

 suboesophageal mass. 



The only organs of special sense which are recognizable 

 in the Lobster and Crayfish are eyes and auditory organs, 

 and a series of specially modified setose appendages which 



