136 ELEMENTARY BIOLOGY. [CHAP. 



being separated by only a very thin membrane from the air 

 contained in the water, loses carbonic anhydride and gains a 

 corresponding amount of oxygen in its course through the 

 branchiae. 



The branchiae are attached partly to the epimera of the 

 thoracic somites, partly to the proximal ends of the thoracic 

 limbs. The epipodites of the limbs ascend between the sets 

 of branchiae which belong to each somite, and separate them. 

 The branchiae which are attached to the limbs must neces- 

 sarily be stirred by the movement of the latter, and hence the 

 exchange of gases between the blood which they contain, and 

 the water must be, to a certain extent, increased, in propor- 

 tion to the muscular contractions which give rise to the 

 movements 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 the 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 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 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 vis- 

 ceral nervous system. It is usually termed the brain or the 

 suprcKKsophageal ganglion. It is connected by two commis- 



