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AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



of the last thoracic segment. The two or three rudimentary 

 gills are also pleurobranchise, and are attached to the epimera of 

 the two or three preceding thoracic segments. 



The number and position of the gills can be expressed by a 

 branchial formula, as follows : 



6 + (ep)+ 6 



[ The numbers refer to the thoracic segments ; pd = podobranchs ; a* = 

 anterior arthrobranchs ; a p = posterior arthrobranchs ; pi = pleurobranchs ; 

 ep = epipodite; r rudiment.] 



The gills are covered with a very thin cuticle, underneath 

 which lies a delicate epithelium. The connective tissue forming 

 the central parts of the filaments is traversed by a network of 

 blood-passages connected with the external and internal channels 

 of the gill-axis. 



The essential part of respiration consists in diffusion of oxygen 

 into the blood contained in the gills, while at the same time carbon 

 dioxide diffuses out into the surrounding water. The hffiinocyanin 

 acts as an oxygen-carrier, since it is able to take a certain amount 

 of that element into loose chemical combination, and parts with 

 it as readily to the tissues. The water in the gill chamber is 

 renewed chiefly by the action of the scaphognathite which bales 

 it out in front, while fresh water enters below and behind. The 

 movements of the thoracic limbs also assist in the production of 

 currents. 



In the young crayfish water is regularly taken into, and ex- 

 pelled from, the intestine by the anus. This anal respiration may 

 possibly also occur in the adult. 



7. Excretory Organs (Fig. 28, B). A pair of kidneys (" green 

 glands ") are situated in the head, one on each side of the 

 oesophageal connectives. Each is a small green body (gl) round 

 and flattened, from the upper surface of which proceeds the ureter, } 



