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 in maintaining their internal environment at a constant concentration of water and dissolved substances, such as salts. Proper balance of the internal environment (homeostasis) of a fish is in a great part maintained by the excretory system, especially the kidney.
The kidney, gills, and skin play an important role in maintaining a fish's internal environment and checking the effects of osmosis. Marine fishes live in an environment in which the water around them has a greater concentration of salts than they can have inside their body and still maintain life. Freshwater fishes, on the other hand, live in water with a much lower concentration of salts than they require inside their bodies.
Osmosis tends to promote the loss of water from the body of a marine fish and absorption of water by that of a freshwater fish. Mucus in the skin tends to slow the process but is not a sufficient barrier to prevent the movement of fluids through the permeable skin. When solutions on two sides of a permeable membrane have different concentrations of dissolved substances, water will pass through the membrane into the more concentrated solution,
