Which are the main excretory substances? Classify animals based on the excretory substance.

(N/A) The major forms of nitrogenous wastes are $Ammonia$, $Urea$, and $Uric \text{ } acid$.
$\Rightarrow$ $Ammonia$ is the most toxic form and requires a large amount of water for its elimination.
- $Uric \text{ } acid$ is the least toxic and can be removed with a minimum loss of water.
Based on the types of excretory substances, animals are categorized into three groups:
$(A)$ $Ammonotelic$ $(B)$ $Ureotelic$ $(C)$ $Uricotelic$
$(A)$ $Ammonotelic$: The deamination of amino acids produces $Ammonia$. It is a poisonous substance removed with the help of water.
- The method of excretion of $Ammonia$ is called the $Ammonotelic$ process.
- $Ammonia$, being readily soluble in water, is generally excreted by diffusion across body surfaces or through gill surfaces (in fish). Kidneys do not play any significant role in its removal.
- e.g., Aquatic insects, $Osteichthyes$, and tadpoles are $Ammonotelic$.
$(B)$ $Ureotelic$: $Ureotelic$ animals mainly excrete $Urea$.
- $Urea$ is a less poisonous nitrogenous substance. It is excreted mainly by terrestrial vertebrate animals.
- In such animals, $Ammonia$ produced by metabolism is converted into $Urea$ in the liver ($2$ molecules of $Ammonia + 1$ molecule of $CO_2 = Urea$) and released into the blood, which is filtered and excreted out by the kidneys.
$\Rightarrow$ Some amount of $Urea$ may be retained in the kidney matrix of some of these animals to maintain a desired osmolarity.
- The process in which excretion occurs in the form of $Urea$ is called $Ureotelism$. e.g., Mammals, $Chondrichthyes$, and adult amphibians.
$(C)$ $Uricotelic$: $Uric \text{ } acid$ is the least poisonous nitrogenous substance.
- In order to minimize the loss of water, these animals excrete $Uric \text{ } acid$ in the form of pellets or paste. $Uric \text{ } acid$ is formed in the liver from $Ammonia$ ($3$ molecules of $NH_3 + 2$ molecules of $CO_2 = Uric \text{ } acid$).
- e.g., Reptiles, birds, insects, and land snails.
- The process by which $Uric \text{ } acid$ is excreted is known as $Uricotelism$.