vii NERVOUS SYSTEM 379 



or organ of propulsion ; (2) a system of out-going 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. j The respiratory organs, it 

 should be observed, are interposed 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 

 opthalmic artery, heart, and dorsal abdominal artery together 

 answer to the dorsal vessel, part of which has become en- 

 larged and muscular, and discharges the whole function of 

 propelling the blood. The horizontal portion of the sternal 

 artery, together with the ventral abdominal, represents 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 ex- 

 posure to the air takes on a bluish-grey tint. This is ow r ing 

 to the presence of a colouring matter called hczmocyanin, 

 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 haemocyanin 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 earthworm, 

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

 by cesophageal connectives. But the ganglia of the ventral 

 nerve-cord are more distinct, and to them the nerve-cells are 

 confined, the longitudinal connectives betw r een them consist- 

 ing of nerve-fibres only. .The brain is complex and supplies 

 not only the eyes and antennules, but the antennae as well : 



