SS2 Biology Lesson Note on Excretory Mechanism in Earthworm; Insect and Mammal

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TOPIC: EXCRETORY SYSTEM II

CONTENT:

(b) Excretory mechanism in:

• • (i) Earthworm
  • (ii) insects
  • (iii) Mammals.

Nephridia is the excretory organ of earthworms. Each segment of the nephridia contains a pair of tubes. Each nephridium is a long coiled tube derived by the growth of ectoderm and opens into the coelomic fluid of the segment anterior to the nephridiopore. The first part is called the nephridiopore, and it has a minute flattered funnel with the upper lip large than the lower lip. The upper lip is formed mainly of a large central cell which is thickly ciliated on the inner surface of the funnel. The lower lip on the other hand consists of a thickened cluster of small cells which are not ciliated. All the cilia beat into the lumen of the tube. The nephrostome leads into an intracellular duct which bears two rows of lateral cilia. Beyond the septum, the tube is narrower and ciliated. It goes into several loops and then becomes the wider brown, ciliated tube which terminates at the nephridiopore. The opening of this pore is regulated by a sphincter. The nephridia of the earthworm are generally supplied with many fine capillaries which ramify among the coils.

EXCRETORY MECHANISM IN EARTHWORM

The earthworm shows the well-developed segmentation that is characteristic of all animals in the phylum Annelida. Although the major nervous, circulatory, and digestive organs are located near the head, more posterior segments contain peripheral structures for all of these systems. These posterior segments are virtually identical to each other. Earthworms are hermaphroditic, possessing both male and female internal reproductive organs.

EXCRETORY SYSTEM IN INSECTS

Tissues in insects produce nitrogenous waste in the form of soluble potassium urate, which is librated into the blood and taken up by the cells lining the malpighian tubules. The tubules are muscular and their writhing movements facilitate the absorption of urate by stirring up the blood. In the cells of the tubule, the potassium urate reacts with water and carbon (iv) oxide (from respiration) to form potassium trioxocarbonate (iv) and uric acid. The former is reabsorbed into the blood to such an extent that the proximal end of the malpighian tubule becomes filled with solid crystals of uric acid. Water is further reabsorbed by the folded walls of the rectal glands so that by the time urine leaves the body, it is very much more concentrated than the blood.

The remarkable ability of insects to conserve water has contributed towards their success as a group. This is largely due to the action of their malpighian tubules and rectal glands. Insects conserve water more effectively than any other group of animals because they do not drink water. They pass out semisolid waste (uric acid), which contains very little quantity of water.

EXCRETORY MECHANISM IN MAMMALS

We shall focus our attention on the formation of urine. The kidneys of mammals accomplish this task by purifying the blood. Some toxic substances such as nitrogenous salt, sodium salt, potassium salt, calcium salt, and urea are removed from the body through this means.

In urine formation, three processes take place:

1. Ultrafiltration
2. Selective re-absorption, and
3. Tubular secretion.

Now, as the blood circulates round the glomerulus, ultra-filtration occurs. Small molecules such as water, urea, mineral salts, sugar, and plasma solutes pass through the one-cell thick walls of the capillaries and the Bowman’s capsule into the capsular space. Bigger molecules like plasma, protein and blood cells cannot pass through the barrier which thus acts as a filter. High pressure in the glomerulus is essential for the filtration process. This is brought about in the following ways:

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