Final answer:
Freshwater decapods like crayfish utilize antennal glands, also known as green glands, to excrete nitrogenous waste and regulate ionic balance, functioning similarly to kidneys.
Step-by-step explanation:
Freshwater decapods such as crayfish have a specialized excretory organ known as the antennal gland or green gland.
These glands play an essential role in the regulation of ionic concentration and the excretion of nitrogenous waste.
The antennal gland is functionally equivalent to a kidney in other organisms and is a distinguishing feature in the anatomy of crustaceans.