82 THE PHYSIOLOGY OF THE COMMON CRAYFISH. 



compounds whicli contain nitrogen, and these, like other 

 waste products, must be eliminated. In the higher 

 animals, such waste products take the form of urea, uric 

 acid, hippuiic acid, and the like ; and are got rid of by 

 the kidneys. We may, therefore, expect to find some 

 organ which plays the part of a kidney in the crayfish ; 

 but the position of the structure, which there is much 

 reason for regarding as the representative of the kidney, 

 is so singular that very different interpretations have 

 been put upon it. 



On tlie basal joint of each antenna it is easy to see a 

 small conical eminence with an opening on the inner side 

 of its summit (fig. 18). The aperture (x) leads by a 

 short canal into a spacious sac, with extremely delicate 

 walls (s), which is lodged in the front part of the head, in 

 front of and below the cardiac division of the stomach (cs). 

 Beneath this, in a sort of recess, which corresponds with 

 the epistoma, and with the base of the antenna, there is a 

 discoidal body of a dull green colour, in shape somewhat 

 like one of the fruits of the mallow, which is known as 

 the g7'een gland (gg) . The sac narrows below hke a wide 

 funnel, and the edges of its small end are continuous with 

 the walls of the green gland ; they surround an aperture 

 which leads into the interior of the latter structure, and 

 conveys its products into the sac, whence they are excreted 

 by the opening in the antennary papUla. The green gland 

 is said to contain a substance termed guanin (so named 

 because it is found in the guano which is the accumulated 



