The excretory organs not only removes the nitrogenous wastes but also maintain water level in the body. On the basis of the nature of the nitrogenous wastes, animals are grouped into three types-ammonotelic, ureotelic and uricotelic.
(a)Ammonotelic Animals : They excrete nitrogen wastes in the form of ammonia. Ammonotelic animals lose more water with ammonia from their body. e.g., aquatic animals such as sponges,coelenterates,crustaceans.
(b) Ureotelic Animals : They excrete nitrogenous wastes in the form of urea. The loss of water from the body with urea is comparatively less in these animals.e.g. cartilaginous,fishes,amphibians and mammals.