In animal kingdom of metabolism there are several strategies used by organisms to eliminate the waste products of metabolism.
I. Excretion by individual cells
In simple organisms individual cells excrete metabolic wastes directly through cell membrane into a body cavity and then the environment.
Ex: Sponges and Hydras
Individual cells excrete wastes into body cavity called spongocoel. Body fluids ejected from spongocoel through osculum into aquatic environment.
Individual cells excrete wastes into body cavity called gastrovascular cavity. Body fluids ejected from gastrovascular cavity through mouth into aquatic environment.
II. Excretion by flame cells
Excretion by flame cells is found in the planarians or flatworms. Planarians have two strands (on each side of the body) of branching excretory tubules that open to the outside through excretory pores.
Tubules along the bulb – like structure are called flame cells, where beating of cilia propels fluids containing metabolic wastes through the system and out the excretory pores.
III. Excretion by nephridia
Excretion by nephrdia is found in earth worms. A typical nephridium consists of a ciliated funnel or nephridiostome which opens into the coelomic cavity. This is followed by a short and narrow tube called the neck which is bent on itself and opens into the main body of the nephridium.
The septal nephridia are arranged in two rows of 40 - 50, attached on either side of the septa, in each segment, behind the 15th segment. The terminal ducts of the nephridia discharge their content into a pair of septal excretory canals which run along the septa.
These nephrindia pickup metabolic wastes from the coelum and blood vessels and expel these wastes through an excretory pore.
IV. Excretion by malpighian tubules
Excretion by malpighian tubules takes place in Arthropods. Arthropods have several excretory systems. Crustaceans have green glands, which is a pair of excretory organs. Green glands collect waste leftover in different parts of the body, including the bloodstream. These are connected by ducts that lead to pores located under antennae where it is released into the outside environment.
Insects which have malpighian tubules which gather waste from blood and puts it into an excretory duct that leads to intensine. Excretion is eliminated in feces.
V. Excretion by kidneys
In more advanced organisms, especially in vertebrates the excretory system is characterized by specialized organs called kidneys.
Inside the kidneys metabolic wastes are removed from blood of the circulatory system and deposited into a system of ducts and storage organs before being expelled to the outside.