ABSTRACT
By the middle of the twentieth century, it was generally believed that digestion proceeds in the cavities of digestive tract under the effect of the enzymes secreted by various digestive glands (cavital digestion) and cell enzymes (intracellular digestion). The discoveries of lysosomes, membrane digestion and the description of symbiont digestion and induced autolysis significantly changed the knowledge and understanding of digestive processes in fishes. In that path, the concept of digestion also underwent a transformation. Considered at first a vegetative function, it is now clear that digestive system and digestive processes play an important role in the function of the animal organism as a whole. Numerous studies have described the protective, metabolic, homeostatic including osmoregulatory function, regulatory and transformational functions of digestive tract in various vertebrates, particularly fish. The
presence of a number of hormones in intestine-acting not only in the limits of alimentary tract but also in the limits of entire organism-has also been proved. These facts allow us to consider the role of digestive system in relation to the functioning of separated organisms as well as in relation to their communities.
