In Hydra, egestion of undigested food and excretion of nitrogenous wastes occur through

In Hydra, undigested residues are egeted from coelentron through mouth while nitrogenous excretory product (ammonia) is removed through general body surface.
Hydra are solitary, sessile, freshwater coelenterates. They are found in lakes, ponds stream. Hydra is an ammonotelic animal i.e., it excretes waste material in the form of ammonia.