Green Glands of Crustaceans
Crustaceans have a pair of renal organs, each of which consists of an internally closed end-sac with a labyrinth, a long coiled nephridial tubule and a bladder which opens into the excretory pore located at the base of either the antennae where it is called antennary glands or the maxillae where it is called maxillary glands.
Figure: Structure of the renal organ and urine formation in the crayfish
The end-sac in many cases is green and hence the name, green gland. Marine forms have a shorter tubule than freshwater forms because the freshwater forms have an extra salt-reabsorbing segment in the tubule.