ANIMAL I Lat. a living being, from anima, current of air, breath of life, soul, animus. soul, mind; from the Skr. root no, to breathe). A representative of one of the two great groups of organisms. the other including plants. The distinction between animal and plant is hard to draw sharply, although the usual differences be tween the higher representatives of the two groups are obvious enough. 3Iost higher ani mals differ from most higher plants in that their food is chiefly solid and organic, in their capacity for locomotion. in their alimentary tube, muscles, nervous system. and sense organs, in their lim ited growth and greater specialization of parts. This list of differences is realty less formidable than it appears; it. resolves itself chiefly into a difference of food, which demands that the animal shall seek the food and be provided with organs for locomotion Imuseles, nervous system, and sense organs) and digestion. The difference in general form of body is due to the different methods of getting the (dissimilar) food. This difference in food (solid and organic, as opposed to fluid and inorganic) serves in a general way to divide even the lower animals from the lower plants. But most animal and plant parasites are alike in requiring liquid, organic food; even green plants use organic food (some in large quantities; see SuxuEw). and all animals require inorganic food.
Locomotion is not a distinguishing characteris tic of animals, first, because great groups of animals are permanently attached; namely, among protozoans, sucto•ia, sponges; among coelenterates, most hydroids and corals; erinoica (sea-lilies) ; bryozoans, barnacles. and most as eidians. Single cases of attached animals are found in other groups. Secondly,bacteria,diatoms,oseil laria, certain unicellular green alga, and many plant "swarm-spores" are more or less locomo tive. In respect to irritability there is little fun damental difference even between the higher ani mals and plants, for plants respond to the same agents as do animals, but less perfectly. The reproductive process is fundamentally the same in the two kingdoms. In their chemical com
position the higher animals differ from most plants; for the former contain no cellulose, whereas the latter are largely built up of it. But 4.ellulose is found also among animals. especially in the test of the tunicates. in their eell-struc and eell-physiology animals are almost in distinguishable from plants. The fundamental living substance, called protoplasm. is substan tially alike in the two kingdoms, and. it is prob able that future studies will make dimmer rather than clearer the line separating them.
The principal functions of animals are connect ed with nutrition, locomotion, sensation and re action, reproduction, and relation to other organ isms. Nutrition involves first the acquisition of food. Food is (I) inorganic—water. oxygen, certain salts; or (2) organic—eithe• vegetable or animal, either dead or living. passive or active. Attached animals depend mostly on dead or on passive living organisms, brought to them in cur rents of water. Those which live on active ani mals must have the most powerful organs of locomotion and sense. Solid food has to be trit urated by teeth or crushing jaws, and digested in a food-canal. The fluids thus obtained pass through the wall of the food-eanal either into the general body spaces or into blood vessels, which carry them to the tissues, where they are assimilated or burned for heat and energy. When the food is exclusively fluid. it may soak through the body wall, as in tapeworms., which have no alimentary tract. The oxygen required passes through the wall of the body. is imbibed with water, or enters through special thin wall-tracts of the body surface known as gills or lungs. The body space or blood vessels carry the oxygen to the tissues, where it is used in combustion and in building up the organic compounds. The waste products of catabolism in the tissues are cast into the body spaces (o• blood vessels) and eliminated, either directly or by special excretory organs. See ANATOMY; ALIMENTARY SYSTEM; RESPIRATORY SYSTEM; :MUSCULAR SYSTEM, and similar articles.