Excretory Adaptations in Other Animals

Each organism had to solve the problem of removing unnecessary metabolic products. In protozoa, such as amoebas and slippers, these products simply diffuse through the cell membrane into the environment, where their concentration is lower. Freshwater protozoa have a special problem - getting rid of excess water, since their protoplasm, being hypertonic in relation to pond water, tends to continuously absorb it.

This would inevitably lead to swelling and rupture of the cell if such protozoa did not have a contractile vacuole - a small bubble lying in the protoplasm, which “pumps” water out of the cell as quickly as water enters it. In Hydra and other coelenterates, direct release of metabolic products occurs through the cell membrane. In larger animals, the release of metabolic products by diffusion is not enough to prevent the accumulation of toxic substances; there is a need for various special excretory devices.

Flatworms have specialized flame cells - single cells that absorb fluid from the spaces around them and secrete it into the excretory (excretory) tubules. The tubules coming from several “flame cells” merge with each other and eventually open out through the excretory pore. The beating of the cilia of the “flame cells” resembles the vibration of a candle flame, hence their name.

In earthworms, in each body segment there is a pair of specialized organs, the so-called nephridia, which have an excretory function. Nephridium, unlike the “flame cells” of flatworms, is a tubule open at both ends; its inner end opens as a whole into a funnel equipped with cilia. Each tubule is surrounded by capillaries, which allows the removal of waste products from the blood.

When the fluid, driven by the beating of the cilia in the funnel, passes through the nephridium, water and substances such as glucose are absorbed back, and the waste products are concentrated and excreted from the body. The excretory system of insects consists of organs called Malpighian tubules, which lie in the body cavity and open into the digestive tract. Metabolic products diffuse from the body cavity into these tubes and are released into the digestive tract, from which they exit along with undigested food.

In all vertebrates, the urinary system is basically the same. In lower vertebrates, the renal tubules open not into Bowman's capsules, but into the body cavity, and thus their excretory organs are intermediate between the nephridia of the earthworm and the kidneys of higher vertebrates.

Vertebrates living in or near the sea have developed special adaptations for removing salt. Bony fish, for example, drink salt water and then secrete salt through their gills. Sea turtles and gulls can secrete salt from drinking seawater using specialized salt glands located in their heads. The ducts of these glands open into the nasal cavities or exit to the surface of the head.

The evolution of the urinary system is complicated by the fact that in many animals some parts of this system are closely related to the reproductive system, so that a number of organs are involved in both functions. This connection is so close that both systems are often considered together under the name of the genitourinary system.