vn RESPIRATORY AND EXCRETORY ORGANS 375 



According to their point of origin the gills are divisible 

 into three sets first, podobranchs, or foot-gills, springing 

 from the epipodites of the thoracic appendages, from 

 which they are only partially separable ; secondly, 

 arthrobranchs, or joint-gills, springing from the articular 

 membranes connecting the thoracic appendages with the 

 trunk ; and thirdly, pleurobmnchs, or wall-gills, springing 

 from the lateral walls of the thorax, above the attach- 

 ment of the appendages. The total number of gills is 

 eighteen, besides two filaments corresponding to vestigial 

 or vanishing gills, which are represented by functional 

 organs in some allied forms. 



The water in the branchial chamber is constantly 

 renewed in the living crayfish by the action of the plate 

 attached to the second maxilla (p. 367), the movement 

 to and fro of which bales out the water in front, and 

 consequently causes fresh water to flow in behind. Thus 

 a fresh supply of water, containing air in solution, is 

 continually being passed over the gills. The fact that 

 the podobranchs are attached to the bases of the limbs 

 must also result in bringing their surfaces more easily 

 in contact with the water when the animal uses its legs 

 in walking. 



The excretory organs differ both in position and in 

 form from those of the earthworm. At the base of each 

 antenna is an organ of a greenish colour, the antennary 

 or green gland (Fig. 95), by which the function of renal 

 excretion is performed. The gland is cushion-shaped, 

 and contains canals and irregular spaces lined by 

 glandular epithelium: it discharges its secretion into 

 a thin- walled sac or urinary bladder (bl), which opens 

 by a duct on the proximal segment of the antenna. The 

 green glands are to be looked upon as corresponding to 

 peculiarly modified nephridia (p. 340). 



The circulatory organs are in a high state of develop- 



