314 ZOOLOGY. 



(4) A median sternal artery, running vertically ventral- 

 wards and dividing into an anterior and posterior median 

 ventral arteries supplying the ventral region of thorax and 

 abdomen respectively. 



(5) A dorsal abdominal artery, supplying the proctodsenm 

 and extensor muscles of abdomen. 



From these arteries the blood passes into the capillaries 

 of the various tissues, and thence into large venous sinuses, 

 of which the main one lies along the ventral surface. From 

 this sternal sinus it passes into the gills, and from thence 

 back by a series of branchio-cardiac canals into the perf- 

 cardial sinus. 



18. Absence of Ccelom, The series of large venous 

 sinuses containing deoxygenated blood, and the pericardial 

 sinus containing oxygenated blood, constitute the anatomical 

 "body-cavity" of the crayfish that is to say, the cavity 

 which serves in dissection as the limit between " alimentary 

 canal " and " body-wall." But since it contains blood, and 

 forms part of the vascular cavities, it is obviously a different 

 thing from the coelom of Vertebrates, or of the earthworm, 

 or the pericardium of the mussel. The coelom in the cray- 

 fish is very much reduced. It is represented by the cavities 

 of the tubular gonads ; the excretory organ and the genital 

 ducts are ccelomic ducts, nephrldia being absent. 



19. The blood is a colourless fluid, containing colourless 

 amoeboid corpuscles. Dissolved in the plasma is a faintly 

 bluish compound called hsemocyanin : this has similar 

 oxygen-carrying properties to haemoglobin, and further 

 resembles it in the fact that a metal copper, not iron, 

 however is an essential constituent of it. 



< 20. Respiration. The gills (fig. 157) are vascular out- 

 growths of the body, on which the cuticle is very thin. 

 Some arise from the epipodites of appendages : these are 

 called podobranchs. Others are outgrowths from the soft 

 articular region at the base of an appendage : these are 

 called arthrobranchs. Others arise from the sides of the 

 body above the articulation of an appendage : these MO 



