vii 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 con- 

 necting the thoracic appendages with the trunk ; and 

 thirdly, pleurobranchs, or wall-gills, springing from the lateral 

 walls of the thorax, above the attachment of the append- 

 ages. 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 development. 



