PHYLUM ARTHROPODA 



513 



I commissure, developed sometimes on the right, sometimes on 

 e left side, its fellow being suppressed. 

 The blood when first drawn is colourless, but after exposure to 

 the air takes on a bluish-gray 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, as a carrier of 

 oxygen from the external medium to the 

 tissues. The hasmocyanin is contained in 

 the plasma of the blood: the corpuscles 

 are all colourless leucocytes. 



The nervous system (Fig. 406) con- 

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

 a ventral nerve-cord, united by cesophageal 

 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 arcki- 

 cerebrum, the eptfiglion of the prostomium, 

 and a secomary brain or syn-cerebfuni 

 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-cesopliageal 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 viscer.al 

 system of nerves (s) supplying the stomach, 

 originating in part from the brain and in part from the oeso^ 

 phageal connectives. 



Sensory Organs. The eyes have the same essential structure 

 as the compound eye of Apus. The chitinous cuticle covering 

 the distal end of the eye-stalk is transparent, divided by delicate 



VOL. I L L 



FIG. 406. Nervous system of 

 Astacus fluviatilis. 



bff. sub-03sophageal gang- 

 lion ; eg. commissural 

 ganglion ; ft, brain ; ., vis- 

 ceral nerve ; sc, oesopha- 

 geal connective ; ?/, post- 

 cesophageal commissure ; 

 IV VIII, thoracic gang- 

 lia ; 1 6, abdominal gang- 

 lia. (From Lang's Com- 

 />i-fi,-i- Anatomy, after 

 Vogt and Yung.) 





