[Photo credit: KDS444. Earthworm_head.svg from wikipedia commons. License: CC-BY-SA 3.0]
An earthworm has a segmented body. Each segment of the body except the first three and the last segment has a pair of tubes called nephridia. The nephridia are the excretory organs of the earthworm.
Each nephridium has a ciliated funnel, the nephrostome at one end, which opens to the outside through a small pore called nephridiophore. This opens into the body cavity of the segment in the front. The nephrostome leads into a long coiled tube made up of:
- a narrow ciliated tube,
- a middle ciliated tube,
- a wide non-ciliated tube, and
- a muscular tube which opens to the exterior via an excretory pore. The nephridium thus opens at both ends.
Mechanism of Excretion in Earthworm
Each nephridium is surrounded by a capillary network. The waste products, mainly urea, are extracted from the blood capillaries surrounding the nephridia. Waste products are also removed from the fluid into the body cavity and passed into the funnel.
Fluid containing waste products moves through the long tubes of the nephridia. Along the way, salts and other substances that are useful to the body are reabsorbed through the walls of the tubes.
The unabsorbed substances, including water, collect in the muscular tubes as urine. The excretory pore relaxes to allow the urine to escape to the exterior. Earthworm gets rid of carbon dioxide during gaseous exchange through the moist body surface or skin.
The earthworm cannot survive in a dry environment. With limits, it can tolerate dry conditions and conserve water by producing very little urine which is hypertonic.
EXCRETORY SYSTEM OF FLATWORM
Flatworm may be free-living or parasitic, and aquatic or terrestrial. Parasitic worms live inside their host's bodies in a fluid environment.
The excretory organs of the flatworms, especially free-living planarians, are the flame cells. In these animals, there are two longitudinal excretory canals with a number of openings to the exterior on the surface. The canals consist of numerous branched tubules. The fine end of the tubules end in a specially hollow structures called flame cells.
In a flame cell, the nucleus is displaced to one side of the cell and the cytoplasm has a large hollow called the cell lumen. The lumen is continuous with the fine tubules. A bunch of flagella hangs down the lumen.
Mechanism of Excretion in Flatworm
The waste products, especially water, ammonia and carbon dioxide diffuse from the surrounding cells into the flame cells. With the aid of flagella, the fluid containing the waste products is propelled into the tubules. From there, the fluid passes into the exterior.
EXCRETORY SYSTEM OF INSECTS
The organs responsible for excretion in insects are the malphigian tubules. These are outgrowth from the exterior portion of the small intestine. They are fine, long and slender.
Insects have the remarkable ability to conserve water so that they can be found in the hottest and driest places on earth. Their ability to conserve water can be attributed to several factors:
- their outer surface is waterproof because of a layer of wax;
- the spiracles prevent water loss from the gaseous exchange surface which is inside the body; and
- they have an extremely efficient excretory system.
The malphigian tubules are found floating between glands and organs in most parts of the thorax and abdomen. The malphigian tubules are found in six groups, each consisting of up to twelve tubules. A malphigian tubule consists of two parts - the distal (free) end and the proximal end (the portion next to the gut).
Mechanism of Excretion in Insects
Nitrogenous waste products and water which are liberated into the haemocoel are absorbed at the distal end of the malphigian tubules. The nitrogenous wastes are converted to uric acid as they pass along the malphigian tubules towards the gut.
A lot of water is also reabsorbed so that by the time the uric acid reaches the proximal end of the tubules, it is changed to solid crystals. In the hind gut, more water is reabsorbed by the rectal gland. Thus, the urine which eventually leaves the body is very concentrated, almost a dry solid.
EXCRETORY SYSTEM OF AMOEBA
Contractile vacuoles are the excretory organs of Amoeba. Water constantly enters the amoebic cell through its selective permeable membrane by osmosis. The excess water collects in a contractile vacuole which is a small sac, lined with a membrane.
The contractile vacuole expands as it is filled with water and then empties its content to the exterior. Thus, the vacuole acts as a kind of water pump for getting rid of excess water. This water has almost no salt in it.
Mechanism of Excretion in Amoeba
Excretory products like ammonia and carbon dioxide, leave the Amoeba by diffusion across the selectively permeable cell membrane to the exterior. This is an adequate mechanism for excretion of metabolic wastes since the surface area to volume ratio of the Amoeba is large.
0 comments:
Post a Comment