XI 



PHYLUM ARTHROPODA 



555 



whole function of propelling the blood. The horizontal portion 

 of the sternal artery, together with the ventral abdominal, 

 represent a 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 

 \cemocyanin, which becomes blue when com- 

 bined 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 con- 

 tained in the plasma of the blood : the 

 corpuscles are all colourless leucocytes. 



The nervous system (Fig. 440) con- 

 sists, like that of Apus, of a brain (g) and 

 a ventral nerve-cord, united by oesophageal 

 connectives (sc). But the right and left 

 halves of the ventral cord have undergone 

 partial fusion, so that the ganglia, and in 

 the abdomen the connectives also, appear 

 single instead of double. Moreover, the 

 brain supplies not only the eyes and anten- 

 nules, 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 dis- 

 tinguish between a primary brain or archi- 

 cerebrum, the ganglion of the prostomium, 

 and a secondary brain or syn-cerebrum 

 formed by the union of one or more pairs 

 of ganglia of the ventral cord with the 

 archi-cerebrum. A further case of con- 

 crescence of ganglia is seen in the ventral 

 nerve -cord, where the ganglia of the last 

 three cephalic and first three thoracic seg- 

 ments have united to form a large com- 

 pound sub-cesophageal ganglion (bg). 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 (s) supplying the 

 gizzard, originating in part from the brain and in part from the 

 oesophageal connectives. 



FIG. 440. Nervous system of 

 Astacus fluviatilis. 

 bg. sub-oesophageal gang- 

 lion ; eg. comtuissural 

 ganglion ; g, brain ; , vis- 

 ceral nerve ; sc, oesopha- 

 geal connective ; y, post- 

 cesophageal commissure ; 

 IV VIII, thoracic gang- 

 lia ; 1 6, abdominal gang- 

 lia. (From Lang's Com- 

 parative Anatomy, after 

 Vogt and Yung.) 



