Unicellular organisms remove waste products through the surface of the body. So, protozoa are released from metabolic products by diffusing them through the membrane. Intestinal and sponges do not have specialized organs or excretory systems. Removal of metabolic products from them is carried out by all body cells by diffusion directly into the water of the environment. The excretory system first appears in flatworms. In them, the bulk of metabolic waste passes into the highly branched intestines and is excreted from the body through the mouth opening. However, some of them enter the system of tubules that perform an excretory function. These tubules are called protonephridia. In flatworms, the excretory organs are protonephridia, or modified skin glands. Two excretory channels stretching along the body and starting with a glandular (secretory) cell in the esophagus. Two channels open at the head end of the body.
In annelids, the excretory organ is metanephridia, which is a ciliated funnel that is connected to long tubules that open outward by excretory pores. By beating cilia, metabolic products are removed from the body, and water and glucose are absorbed into the capillaries entwining the nephridium. The earthworm has a pair of nephridia in each segment, while other representatives have fewer. In mollusks, the excretory system is represented by one or two kidneys, conducting tubules and excretory pores. The kidney is connected with the circulatory system, absorbing the final decay products from the blood. The mechanisms of excretion in arthropods are very diverse. This is due to the fact that arthropods have mastered the most diverse habitats – from marine to terrestrial.
Many crustaceans have specialized organs of excretion – green glands located at the base of the antennas.
They are an annular sac with a greenish glandular chamber and a channel going into the bladder. Urine builds up and pours out.
The excretory system of the vertebrates is represented by paired kidneys, which secrete liquid waste products from the blood when it reaches them through the capillaries. The ureter departs from each kidney, which opens into the bladder. From the bladder, metabolic products are removed through the urethra. The kidney consists of a network of numerous renal tubules, pierced by a dense network of capillaries. Due to diffusion, liquid waste products from the blood enter the kidneys.
The excretory system of fish is represented by two ribbon-shaped red – brown trunk kidneys located in the body cavity between the swimming bladder and spine, from which two ureters depart. Blood brings decay products and through the ureters urine flows into the bladder, and is removed from it through a special hole behind the anal.
The excretory system of amphibians and reptiles is almost the same.
In amphibians, trunk kidneys are located in the body cavity along the sides of the spine. A frog can lose up to 1/3 of body weight daily with urine and skin.
In reptiles – in the area of the pelvic bones, the pelvic kidneys are located. The glomeruli of the kidneys are not large and filter water from the blood less. Substances excreted by the kidneys from the blood through the ureters enter the cloaca, the enlarged part of the hind gut, and from it into the bladder. After filling the bladder, urine enters the cloaca and is removed to the outside.
The pelvic kidneys of birds are quite large, this is due to a more intense metabolism, two ureters also open from them, opening into the cloaca. From the cesspool, part of the water contained in the urine is absorbed back. Urine in birds is not watery, but looks like a liquid white slurry. Urine concentration is high because metabolism in birds enhanced. The birds do not have a bladder. Urine in the organs of excretion does not accumulate, but is almost immediately eliminated. This is a tool to lighten the flight weight of birds. The presence of cesspools in birds is evidence of their kinship with reptiles.
In mammals, pelvic kidneys serve as the main excretory organ. They are bean-shaped and located in the lumbar region on the sides of the spine. The kidneys help maintain water-salt and acid-base balance. From the kidneys through two ureters, urine flows into the bladder, from which it is removed through the urethra. In addition, in mammals, metabolic products can be secreted through the skin (sweat glands) and other organs.