the crayfish 



257 



fill 



like that of the frog, which is a separate part of the ccelom. 



A blood-vascular system in which, as in the crayfish, the 



blood on leaving the arteries bathes the organs of the body 



is said to be open. One in which, as in the worm and the 



frog, it is carried through the organs in capillaries which 



lead direct to veins is said to be closed. The blood of the 



crayfish is a clear fluid, which contains white corpuscles and 



clots readily — an obvious advantage to an animal whose 



open vascular system causes it 



to bleed freely from any wound. 



An organic compound of copper, 



known as hcemocyanin, which is 



dissolved in it, plays the same 



part as haemoglobin, taking up 



oxygen in the respiratory organs 



and parting with it to the tissues. 



In the oxidised condition it is of 



a blue colour and tinges the 



blood. 



The respiratory apparatus of 

 the crayfish is con- 

 2?!anf ry tain ed in the gill- 

 chambers. The gills 

 are branched, thin-walled struc- 

 tures, standing upon the coxo- 

 podites of the thoracic limbs and 

 the inner wall of the gill-chamber. 

 In them the blood circulates 

 and exchanges its carbon dioxide 

 for the oxygen which is dissolved 

 in the water that is kept flowing 

 through the chamber by the 

 action of the second maxilla.' This limb is held firm by the 

 curved end of its endopodite, which fits into a groove upon 

 the mandible at the base of the palp, while the exopodite 

 or scaphognathite, flapping at the rate of sixty strokes a 

 minute, bales water forwards, out of the gill-chamber and 

 under the opening upon the antenna of the green gland, 

 whose excreta it thus sweeps away with the foul water from 

 the gills. By this action fresh water is drawn into the 

 chamber between the bases of the legs. No doubt the blood 



Fig. 164. — A podobranch of 

 the crayfish, seen from be- 

 hind. 



Base ; cf., coxopodite ; gill', lam., 

 lamina; sb., setobranch or 

 tuft of coxopoditic setas ; stm., 

 stem. 



