Bacteria have simple digestive systems whereby food is ingested and wastes are eliminated from the body through the cell membrane by use of the contractile vacuole by the process of simple diffusion. On the other hand human beings have a more developed digestive system with various organs being used to remove wastes for instance the skin, liver and anus. Therefore the main distinction between bacterial excretion and human digestive systems are the organs that are present in the humans that facilitate the removal of various wastes. Each organ in humans is adapted for excreting specific wastes for instance the liver is adapted for the removal of urea from the body. This is not the same case for bacteria which removes most of the wastes together. Specialization of organs for removal of wastes is not found in bacteria because of their simple body plan.
Bacteria such as amoeba eliminate nitrogenous wastes in form of ammonia which requires a lot of water for removal while humans excrete their nitrogenous wastes in the form of urine from the kidney nephrons. ( Bunnell, 1984).
Grasshoppers are underclass Insecta group of animals. Their digestive system differs from that of the human beings in some respects. First of all, insects possess structures called malpighian tubules that collect wastes such as ammonia, uric acid and faecal pellets from the gut and haemolymph for elimination. The malpighian tubules are not found in humans instead we have the liver, skin and anus for eliminating the waste products of digestion. In insects the nitrogenous wastes are eliminated in the form of uric acid while in the human beings the nitrogenous wastes are eliminated in form of Urine and urea in the sweat.
Fish have a very simple digestive system as compared to that of the humans. The fish have mouth sucking parts for ingesting food unlike the teeth in the humans.
The fish use their gills for removing some wastes from their bodies. Their nitrogenous wastes are in the form of ammonia and require water as they diffuse out of the gills. Other wastes in the fish are removed by the use of the kidneys that are adapted to filter wastes from the blood. Fish have structures called caeca in their digestive system and this structures absorb water before wastes are removed via the anal opening. These structures are not found in the complicated systems of humans.( Helfman , Collette & Facey 1997)
This is not the same case in the humans because the humans do not have gills and they do not use diffusion to remove their nitrogenous waste. Unlike ammonia the humans excrete urine from their well developed kidneys. The fish do not use their skin for excretion like the humans do. Fishes also do not remove solid wastes from digestion like the humans do via the anus.
The bear is a mammal in the family Ursidae. Their digestive system is similar to that of the human being. Differences only come in the They differ from human beings in terms of their dentition. The dentition in the Bear is different from that of the human being because of the food substances the two feed on. The digestive tract of a bear differs from that of the human being in terms of length particularly for structures such as the intestines. The human colon is more elaborate than that of a bear. This follows that the faecal excrement of the human will differ from that of the bear.
References
Bunnell, Fred (1984). Macdonald, D. ed. The Encyclopedia of Mammals. New York: Facts on File. pp. 87.
Helfman G., Collette B., & Facey D.: The Diversity of Fishes, Blackwell Publishing, p 3, 1997, Protists. Web.