xxvn NERVOUS SYSTEM 341 



The blood when first drawn is colourless, but after ex- 

 posure to the air takes on a bluish-gray tint. This is owing 

 to the presence of a colouring matter called hamocyanin, 

 which becomes blue when combined with oxygen ; it is a 

 respiratory pigment, and serves, like haemoglobin, 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. 



The nervous system consists, like that of Polygordius, of 

 a brain (Fig. 86, g) and a ventral nerve-cord {bn), united by 

 oesophageal connectives. But the ventral nerve-cord is 

 differentiated into a series of paired swellings or ganglia to 

 which the nerve-cells are confined, united by longitudinal 

 connectives. 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 

 brain proper. Hence we have to distinguish between a 

 primary brain or archi-cerebrum, the ganglion of the prosto- 

 mium, and a secondary brain or syn-cerebruvi formed by the 

 union of one or more pairs of ganglia of the ventral cord 

 with the archi-cerebrum. A further case of concrescence of 

 ganglia is seen in the ventral nerve-cord, where the ganglia 

 of the last three cephalic and first three thoracic segments 

 have united to form a large compound sub-oesophageal 

 ganglion. All the remaining segments have their own 

 ganglia, with the exception of the telson, which is supplied 

 from the ganglion of the preceding segment. There is a 

 visceral system of nerves supplying the stomach, originating 

 in part from the brain and in part from the oesophageal 

 connectives. 



The eyes have a very complex structure. The chitinous 

 cuticle covering the distal end of the eye-stalk is transparent, 



