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| Excretion and Osmoregulation in Earthworm |
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| The earth worm belongs to the phylum Annelida. This group of higher invertebrates possess a number of highly developed organs and organ-systems for performing various vital functions. |
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| Earthworm |
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| One such elaborate system is the excretory system which consists of small, coiled tubes with walls that are glandular and richly supplied with blood vessels. These coiled tubular excretory structures are known as nephridia. They occur in all the segments of the body starting from the third segment downwards. Based on their location three kinds of nephridia can be distinguished. |
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| These lie attached to the inner side of the body wall in all segments except the first two. |
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| These lie attached to both sides of the septa behind the 15th segment. |
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| Septal Nephridia |
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| These are located only in the 4th, 5th and 6th segment. |
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| The septal nephridia may be considered as a typical nephridia for detailed description. |
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| 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 main body of the nephridium consists of a short straight region called the straight lobe and a long spirally twisted loop. The entire length of the nephridium contains the nephridial tube which leads into the terminal nephridial duct. The cells lining the nephridiostome and the nephridial tube bear cilia in some lengths. The lashing movements of the cilia help in the flow of the excretory matter. |
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| 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. The septal excretory canals open into a pair of intestinal excretory ducts. These ducts in turn open into the cavity of the gut through short ductules. Thus the waste is finally discharged into the gut. The septal nephridia are responsible for preventing loss of water. The integumentary nephridia are the smallest of the three types and are most numerous 200-250 in each segment. These nephridia discharge the waste products outside the body through small openings on the body wall called nephridiopores. The pharyngeal nephridia occur as paired tufts surrounding the alimentary canal in the 4th, 5th and 6th segments. A large number of nephridia are grouped together to form each tuft. The terminal ducts of all the nephridia unite to form a common excretory duct. The three pairs of common excretory ducts discharge their wastes into the pharynx and buccal chamber. |
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| Thus it is only the integumentary nephridia which open directly to the exterior and so they are called exonephric nephridia. As the septal and pharyngeal nephridia discharge their waste products into the alimentary canal, they are called enteronephric nephridia. |
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| Since the nephridia are richly supplied with capillaries, nitrogenous wastes are removed from the blood. The rhythmic beating of the cilia of the nephridiostomes and tubules of the septal nephridia drives a steady stream of coelomic fluid containing metabolic waste materials, such as urea, ammonia and remains of dead cells, into and through the nephridial tube into the septal excretory canals and supra intestinal excretory ducts. The excretory wastes are then finally discharged into the gut. The pharyngeal and integumentary nephridia extract only waste matter from the capillaries with which they are richly supplied. Thus water is conserved by these nephridia. |
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