The organs of the body are composed mainly of these tissues, and present the greatest diversity of structure and function; but they may be roughly arranged, according to their functions, into four groups: organs of nutrition, such as the digestive, circulatory, and respiratory organs; organs of reproduction, organs of motion, and organs of relation, such as the nervous system and sense-organs, and organs of defense or protection, such as horns or spurs. Except among the lower and simpler groups the individuals are not organically united, as in plants, into a community, although such communities as pairs, or flocks, or herds, are frequent. In such a community as a hive of bees the different individuals are specialized for the good of the whole, and are unable to exist apart; and the community is as real as in the case of a plant, although the connection is not material, but purely ideal.
Physiologically, animals are characterized by the fact that, with few exceptions, they are able to receive solid food into a definite internal digestive cavity, in which it is digested, and then absorbed through the wall of the cavity. They absorb oxygen from the surrounding medium, air or water, and, in addition to certain inorganic substances, take into their bodies, as their proper nutritive material, complicated protein compounds, which they derive either directly or indirectly from plants. Through the oxidation of these compounds they form substances of a simpler chemical structure, such as carbonic acid, water, and ammonia, and discharge these through their bodies as waste. Since the sum of the chemical changes which take place in the animal is the breaking down of the highly-complex protein-molecules, derived from plants, into the simpler molecules of water, ammonia, and carbonic acid, it results that the potential energy thus set free is shown by the animal as "vital force," and the body of the animal is therefore a magazine or storehouse of force which may manifest itself as animal heat or light, or as sensible motion, or nervous disturbance; an animal, then, is an organism which has the power to change the potential energy of vegetable protein by oxidation into "vital force," which may manifest itself as animal heat, or, in the case of many marine animals, as light, or by peculiar disturbances of the nerves and muscles, organs which are peculiarly diagnostic of animals. The changes of the muscles result in motion, either of the animal as a whole or of the various parts in relation to each other. The structure and functions of the nervous system of one of the higher animals are so entirely different from any other phenomena, that they seem to be sui generis and peculiar; but we must not forget that there are true animals which entirely lack a nervous organization, and that in the history of each individual, as well as in the history of the animal kingdom, we may pass without any considerable break from animals with a complicated system of nerves and sense organs to animals which give no evidence of conscious life, and are no more sensitive than ordinary plants. The nervous system of an animal may be roughly described as a regulative apparatus by which the vari-