82 THE PHYSIOLOGY OF THE COMMON CRAYFISH. 
compounds which 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, hippuric 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 the 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 isa 
discoidal body of a dull green colour, in shape somewhat 
like one of the fruits of the mallow, which is known as 
the green gland (gg). The sac narrows below like 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 papilla. The green gland 
is said to contain a substance termed guunin (so named 
because it is found in the guano which is the accumulated 
