II.] THE CRAYFISH AND LOBSTER. 183 



Each branchia is somewhat like a bottle-brush, having a 

 stem beset with numerous filaments; and the blood con- 

 tained in the vessels of the latter, 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 exclusively thoracic, being attached 

 partly to the inter-articular membranes between the appen- 

 dages and the body-wall and partly to the proximal ends of 

 the limbs themselves. The last thoracic appendage is gill-less, 

 and the branchia present in its vicinity in the Crayfish differs 

 from the rest in being attached to the epimeral wall of the 

 thorax; the Lobster has, in addition, three such gills on 

 either side fully developed and functional : all or most of 

 these are represented in the Crayfish by short filamentous 

 rudiments, no longer functional as branchiae. The epipo- 

 dites 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 necessarily 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 proportion to the 

 muscular contractions which give rise to the movements 

 of the limbs and the consequently increased formation of 

 carbonic anhydride. 



The excretion of nitrogenous waste goes on in the two 

 large green glands which lie in the cephalon, close to the 

 bases of the antennae. Each gland encircles the neck of a 

 large thin-walled muscular sac which opens by a short 

 canal upon the ventral face of the basal joint of the antenna. 

 The gland itself consists of a coiled tube, lined by a 

 large-celled epithelium and abundantly supplied with blood 

 vessels. 



