Important notes on excretion:
Excretion is a process by which the by-products of cellular metabolism are so treated that they take no further part in the metabolism. These by-products are removed from the body in aqueous solution. Based on the nitrogenous products they excrete, animals can be classified into three groups:
In these animals, nitrogen is excreted predominantly in the form of ammonia, e.g., crustaceans, polychaete annelids, molluscs, teleosts and tadpoles of amphibia.
The animals in which nitrogen is excreted in the form of urea are known as ureotelic animals, e.g., elasmorbranch, amphibians and mammals.
These animals excrete the nitrogenous wastes in the form of uric acid, e.g., insects, gastropods, lizards, snakes and birds.
Excretion in Invertebrates:
In most unicellular organisms, excretion is a simple process of diffusion into the surrounding medium. In other organisms, contractile vacuole may also help in excretion. In sponges and coelenterates, waste is diffused through epidermal cells to the surrounding water or into the canals and through gastrovascular cavity by the en- dothermal cells. In flatworms, there are definite excretory organs, the flame-cells (protonephridia) which discharge the waste outside.
In annelids, there are nephridia to remove the waste matter. Insects have many malpighian tubules which pick-up waste substances from the body of diffusion and pass them into the digestive tracts. In other arthropods, especially the crustaceans, the excretion is performed by a pair of antennary or maxillary glands. Molluscs have one or more pairs of kidneys or nephridia that remove the wastes from the coelom and the blood. In echinoderms, excretion is performed by amoebocytes.
Excretion in Vertebrates:
In vertebrates, the following organs are concerned with the excretion of body Wastes:
It removes excess water, salts and CO2.
They remove CO2 and water.
It excretes bile pigments.
They remove excess salts in marine fish.
They remove excess of water, nitrogenous wastes, salts and other materials in the form of urine. 75 - 80% excretion of wastes in man and other mammals takes place through the kidneys.