BLOOD VESSELS 2O9 



respiratory organs and parting with it to the tissues. In the 

 oxidised condition it is of a blue colour and tinges the blood. 



RESPIRATORY ORGANS 



The respiratory apparatus of the crayfish is contained in the 

 gill-chambers (Fig. 141). The gills (Fig. 142) are branched, thin- 

 walled structures, standing upon the coxopodites 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 



ah. 



arb. 



scg. 



Fig. 141. — The forepart of the body of a crayfish, viewed from the right-hand 

 side, with the legs and the branchiostegite cut away and the gills displayed. 



arb., Arthrobranchiae ; ep., epipodite of the first maxilliped ; pbr., podobranchice ; plb., pleurobranchia ; 



scg., scaphognathite. 



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, and when the oxygen concentration in the water is reduced, 

 the baler flaps faster, so that the supply of oxygen is kept up. 

 No doubt the blood in the branchiostegite is oxygenated through 

 the thin inner wall of that organ. 



The gills receive different names according to their position. 

 Those which are attached to the epipodites of the limbs are known 



