vii NERVOUS SYSTEM 379 



outgoing channels, the arteries, which carry the blood 

 from the heart to the body generally ; and (3) a system 

 of returning channels some of them, the sinuses, mere 

 irregular cavities, others, the veins, with definite walls : 

 these return it from the various organs back to the heart. 

 The respiratory organs, it should be observed, are inter- 

 posed in the returning current, so that blood is taken 

 both to and from the gills by veins. 



Comparing the blood-vessels of the crayfish with 

 those of the earthworm (Figs. 81 and 83), it would seem 

 that the ophthalmic artery, heart, and dorsal abdominal 

 artery together answer to the dorsal vessel, part of 

 which has become enlarged and muscular and discharges 

 the whole function of propelling the blood. The ventral 

 thoracic artery, together with the ventral abdominal, 

 represent the * main ventral vessel, while the vertical 

 portion of the sternal artery is a commissure, developed 

 sometimes on the right sometimes on the left side, its 

 fellow being suppressed. 



The blood when first drawn is colourless, but after 

 exposure to the air takes on a bluish-grey tint. This is 

 owing to the presence of a colouring matter called 

 hcemocyanin, which becomes blue when combined with 

 oxygen ; it is a respiratory pigment, and serves, like 

 haemoglobin (pp. 107 and 339), as a carrier of oxygen 

 from the external medium to the tissues. The haemo- 

 cyanin is contained in the plasma of the blood : the 

 corpuscles are all leucocytes (pp. 105 and 336). 



The nervous system consists, like that of the earth- 

 worm, of a brain (Fig. 93, g) and a ventral nerve-cord 

 (bm), united by cesophageal connectives. But the 

 ganglia of the ventral nerve-cord are more distinct, and 

 to them the nerve-cells are confined, the paired longi- 

 tudinal connectives between them consisting of nerve- 

 fibres only. The brain is complex, and supplies not 



