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The main organ in the excretory system of all Osteichthyes is the kidney. They are complex, and within them is the nephrons. The nephrons form in segments much like an Earthworm. The corpuscle is part of the nephron that filters blood, and the tubule send back water and solutes (nutrients) into the blood. The bad and useless componets of the blood are sent to the ureter which is where urine is removed from the body. Solid wastes are processed through the digestive tract, and removed from the body out of the anus.
In fishes, some excretion also takes place in the digestive tract, skin, and especially the gills (where ammonia is given off). Compared with land vertebrates, fishes have a special problem
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In Fugo chrysops, the distinction between the kidney and the head-kidney is not clear. A pair of ureters leave the posterior end, and they join in front of the small urinary bladder. Corpuscles of Stannus lie on the dorsal side of the junction of ureters. In typical fresh-water fishes, the heterotopic thyoid follicles are often found in the kidney or the head-kidney. In general, the active and fast swimming fishes have numerous and better developed glomeruli than sluggish fish have. Beyenbach and Baustian (1989), Beyenbach and Dantzler (1990), Baustian and Beyenbach (1993), Beyenbach (1995) and Beyenbach and Lui (1996) have provided a wealth information on the comparative physiology of the proximal tubule. As the first major tubule along the length of the nephron, the proximal tubule is invariably present in all vertebrate kidney. It performs most renal functions in glomerular kidneys, and virtually all transport functions in aglomerular kidneys. Aglomerular kidneys secretes magnesium and sulphate ions, an obvious adaptation to marine conditions. Nevertheless, some aglomerular fish can now tolerate dilute brackish water or even fresh water. Osteichthyes release their nitrogen as ammonia. Blood waste are filtered by kidney. And some waste is diffused by the gills. Salt water osteichthyes are constantly secreting water through their skin. They tend to drink a lot of water

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