ANIMAL. The word "animal° being de rived from anima, breath, soul, suggests the dis tinction popularly accorded to animals as con trasted with plants. Linne said that plants grow and live, but that animals grow, live, and feel. As will be seen below, however, animals do not fundamentally and in their simplest forms dif fer from the simplest plants, as both are con stituted of protoplasm, which is equally contrac tile in both kingdoms, and we are coming more and more to speak of living beings as organ isms. All organized beings agree in being formed of protoplasm, "the physical basis of life." Strictly speaking, however, an animal is a living organism, the protoplasm of which does not secrete a cellulose cell-wall, and the exist ence of which requires proteid material obtained from the living or dead bodies of existing plants or animals.
Differences between Plants and Animals. — It is difficult, when we consider the simplest forms of either kingdom, to define what an animal is as distinguished from a plant, for it is impossible to draw hard and fast lines be tween them. In defining the limits between the animal and vegetable kingdoms our ordinary conceptions of what a plant or an animal is will be of little use in dealing with the lowest forms of either kingdom. A horse, fish or worm differs from an elm-tree, a lily, or a fern in hav ing organs of sight, of hearing, of smell, of locomotion and special organs of digestion, cir culation and respiration, but these plants also take in and absorb food, have a circulation of sap, respire through their leaves, and some plants are mechanically sensitive, while others are endowed with motion,—certain low plants, such as diatoms, etc., having this power. In plants the assimilation of food goes on all over the organism, the transfer of the sap is not confined to any one portion or set of or gans as such. It is always easy to distinguish one of the higher plants from one of the higher animals. But when we descend to animals like the sea-anemones and coral-polyps, formerly called "zoophytes,° so striking is the external similarity between the two kinds of organisms that early observers regarded them as "animal flowers.° Sponges until a very late day were regarded as plants. So-called plants, as Bac terium or Bacillus and their allies, and so-called animals, as Protamceba, or certain monads, which are simple specks of protoplasm without genuine organs, may be referred to either king dom. Indeed, a number of naturalists, notably
Haeckel, relegate to a neutral kingdom (the Protista, q.v.), certain lowest plants and ani mals. Even the germs (zoospores) of monads like Uvella and those of other flagellate in fusoria may be mistaken for the spores of plants; and there are certain flagellate infusoria so much like low plants (such as the red-snow or Protococcus), and the slime molds (Myx omycetes) in the form, deportment, mode of reproduction and appearance of the spores, that even now it is possible that certain organisms placed among them are plants. It is only by a study of the connecting links between these lowest organisms, leading up to what are un doubted animals or plants, that we are enabled to refer these beings to their proper kingdom.
As a rule, plants have no special organs of digestion or circulation and nothing approach ing to a nervous system. They differ from ani mals in their metabolic processes. Most plants absorb inorganic food, such as carbonic acid gas, water, nitrate of ammonia, and some phos phates, silica, etc., all of these substances being taken up in minute quantities. Low fungi live on dead animal matter and promote the process of putrefaction and decay, but the food of these organisms is inorganic particles. The slime molds, however, envelop the plant or low ani mals much as an amoeba throws itself around some of the living plant and absorbs its proto plasm; but Myxomycetes, in their manner of taking food, are an exception to other molds and are now regarded as animals. The lowest animals swallow other living animals whole or in pieces; certain forms, like Amceba (q.v.), bore into minute alga; and absorb their proto plasm; others engulf silicious-shelled plants (diatoms), absorbing their protoplasm. No animal swallows silica, lime, ammonia or any of the phosphates as food. On the other hand, plants manufacture or produce from inorganic matter starch, sugar and nitrogenous substances which constitute the food of animals. During assimilation plants absorb carbonic acid and in sunlight exhale oxygen; during growth and work they, like animals, consume oxygen and exhale car bonic acid.