Terrestrial adaptation necessitated the production of lesser toxic nitrogenous wastes like urea and uric acid for the conservation of water. Mammals and many terrestrial amphibians mainly excrete urea and are called uriotelic animals.

Ammonia produced by metabolism is converted into urea in the liver of these animais and released into the blood, which is filtered and excreted out by the kidneys.

Some urea is retained in kidney in order to maintain osmolarity, reptilelos birds, land snails and insects excrete nitrogeneous waste, as uric acid in the form of pellet or paste with a minimum loss of water and are called uricotelic animals. Conversion of ammomia to uric acid and its subsequent elimination requires lesser amount of water.

Hence, due to less availability of water on land, and in order to minimise water loss, lerrestrial organism adapted themselves accordingly.