VI NERVOUS] SVSTEM 365 



body generally ; and (3) a system of returning channels — 

 some of them, the sinuses, mere irregular cavities, others, 

 the veins, with defitiite walls — these return it from the 

 various organs back to the heart. 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, 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 muscu- 

 lar, and discharges the whole function of propelling the 

 blood. The horizontal portion of the sternal artery, together 

 with the ventral abdominal, represent the 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 owing 

 to the presence of a colouring matter called hmmocyaniit, 

 which becomes blue when combined with oxygen ; it is a 

 respiratory pigment, and serves, like haemoglobin (pp. 107 and 

 330), as a carrier of oxygen from the external medium to the 

 tissues. The haemocyanin is contained in the plasma of the 

 blo&d : the corpuscles are all leucocytes (pp. 105 and 328). 



The nervous system consists, like that of the earthworm, 

 of a brain (Fig.' 86, g) and a ventral nerve-cord ibn), united 

 by oesophageal connectives. But the ganglia of the ventral 

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

 confined, the longitudinal connectives between them consist- 

 ing of nerve-fibres only. The brain supplies not only the 

 eyes and antennules, but the antennae as well, and it is found 

 by development that the two pairs of ganglia belonging to 

 the antennulary and antennary segments have fused with the 



