Chap. XII.] THE CRAYFISH. 247 



ventral nerve-chain, and enters the sternal sinus within the 

 endophragmal system. Thence one branch runs forward, sup- 

 plying the ventral region and limbs of the cephalo-thorax so far 

 as the mouth, another backward to supply the ventral region and 

 limbs of the abdomen. 



The arteries break up into capillary ramifications. From 

 these, however, the blood is not collected into veins, as in the 

 vertebrate, but passes into sinuses irregularly disposed among 

 the internal organs. After passing through these sinuses it 

 eventually reaches the sternal sinus (s. s.) beneath the cephalo- 

 thorax. Thence it passes by afferent vessels (a. v. in Fig. 77, B., 

 which shows diagrammatically the circulation in the thorax) to 

 the gill-plumes ; returning by efferent vessels (e. v.) to the base 

 of the gill, whence it is carried to the pericardial sinus (p. s.) by 

 branchio-cardiac vessels (b. c. v.). Thus the blood is aerated on 

 its way to the heart. In the cod-fish it is aerated in the gills on 

 its way from the heart. 



The blood itself is a clear colourless fluid, which coagulates 

 to form a clot. It contains colourless nucleated amoebiform 

 corpuscles. 



The Green Glands. The excretory organs by which nitro- 

 genous waste is eliminated from the blood of the crayfish are 

 the green glands (Fig. 75, gr. gl., and Fig. 77, C.). On the basal 

 joint of the antenna there is a small conical prominence (77, C., 

 c.p.), with an aperture on the inner side of its summit. This 

 leads into the thin-walled sac (5.), which may be regarded as the 

 much-expanded end of an immensely convoluted tube, which 

 ends blindly in a triangular yellowish-brown lobule. This con- 

 voluted tube forms the body of the gland (gr. gl.) The secretion 

 contains large quantities of uric acid, with very small traces of 

 guanin a substance analogous in some respects to uric acid, but 

 less highly oxidised. 



The Nervous System. The nervous system of the crayfish 

 presents us with a chain of ganglia running along the ventral 

 aspect, connected by commissures. 



