XII.] THE FRESH-WATER CRAYFISH. 131 



of the limbs and the consequently increased formation of 

 carbonic anhydride. 



The mode and place of the excretion of nitrogenous 

 waste is not yet clearly made out, but it seems probable 

 that two large green glands which lie in thfe cephalon close 

 to the bases of the antennae are renal organs. Each gland 

 encircles the neck of a large thin-walled sac which opens 

 by a short canal upon the ventral face of the basal joint 

 of the antenna. 



The nervous system consists of a chain of thirteen 

 ganglia — 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 integuments ; and of a 

 visceral nervous system, developed chiefly upon the stomach. 



Of the thirteen ganglia, the most anterior 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 sup7-aceso~ 

 phageal 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 svhcesophageal ganglion. This occu- 

 pies the region of the hinder part of the cephalon and the an- 

 terior part of the thorax, and gives off nerves to the maxills 

 and the three pair 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 between the abdominal 

 ganglia are single ; but, in the thorax, the commissures are 

 double, and the ganglia themselves show more or less evident 

 indications of being double. And there is reason to believe 

 that these thirteen apparent ganglia really represent twenty 

 pairs of primitive ganglia, one pair for each somite ; the ' 



9-2 



