Functions of nutrition--Assimilation and disassimilation--Plasmodoma and plasmophaga--Phytoplasm and zooplasm--Plasmodomism of plants--Chlorophyll granules and nitro-bacteria--Plasmophagism of fungi and animals--Metasitism (conversion of metabolism)--Nutrition of the monera (chromacea, bacteria, rhizomonera)--Nutrition of the protophyta and metaphyta (cell-plants and tissue-plants)--Nutrition of the metazoa--- Gastræa theory--Gastro-canal system of the cœlenteria (gastræads, sponges, cnidaria, platodes)--Nutrition of the cœlomaria (digestion, circulation, respiration, evacuation)--Saprositism--Parasitism--Symbiosis. The wonder of life which we call, in the widest sense of the word, "nutrition" is the chief factor in the self-maintenance of the organic individual. It is always bound up with a chemical modification of the living matter, an organic metabolism (circulation of matter), and a corresponding circulation of force. In this chemical process plasm is used up, built up afresh, and once more disintegrated. The metabolism which lies at the root of this chemistry of food is the essential feature in the manifold processes of nutrition. A large part of the several nutritive processes are explained without further trouble by the known physical and chemical properties of inorganic bodies; for another part of them we have not yet succeeded in doing this. Nevertheless, all impartial physiologists now agree that it is possible in principle, and that we have no reason to introduce a special vital principle. All the trophic (nutritive) processes, without exception, are subject to the law of substance. In all the higher plants and animals the chemical process of metabolism, with the stream of energy that accompanies it, is a very complex vital activity, in which many different functions and organs co-operate with the common aim of self-maintenance. As a rule, they are distributed in four groups--namely: (1) Intussusception of food and digestion: (2) distribution of the food in the body, or circulation; (3) respiration, or exchange of gases; and (4) excretion of unusable matter. In most of the histona, either tissue-plants or tissue-animals, a number of organs are differentiated for the accomplishment of these tasks. At the lower stages of life this division of labor is not found, the entire process of nutrition being accomplished by a single layer of cells (lower algæ, gastræads, sponges, lower polyps). In the protists, again, it is the single cell that does all these things itself; in the simplest cases, the monera, a homogeneous plasma-globule. As a long gradation uninterruptedly unites these lowest forms of nutrition with the more complicated forms, we must regard the latter no less than the former as physico-chemical processes.