Locomotion involves locomotive apparatus of divers kinds, jets of water, suckers and contract ile tubes, lashes. tails, cilia. paddles. fins, wings, and legs. It involves also muscles and a nervous system to control them.
All the protoplasm of the living body is irrita ble, but parts of the surface are told off as areas of special sense; for contact, hearing. taste, smell, sight, and temperature. To receive these impres sions and to set in action appropriate movements, the central nervous system has become special ized. In the definite reactions which accompany particular situations to the world external to the animal lie the first evidences of a "psychic life." All sessile animals are characterized by lack of many sense-o•gans, reduction of muscular and nervous systems, and reduction of instincts.
Owing to accidents, the number of individuals tends constantly to diminish, yet it must be maintained. The single way that organisms have of making good losses or increasing their numbers is by dividing; this is the essence of reproduction (q.v.). Of especial significance is
the fact that in all groups of animals the bits which have been constricted off (gametes) from time to time unite in pairs to form zygotes before going on with their development. In all reproduction, the dividing individuals give rise to two incomplete individuals, except in the ease where the division separates a "germ cell" from the body that carried it. The divided pieces or the germ cells are imperfect representatives of the species: they must or "develop" to produce the adult condition. See EMBRYOL OGY; REPRODUCTION.
The relations of animals to other organisms are varied. Many animals, especially in the higher groups, care for their young. pro tect themselves from their enemies by conceal ment or by flight; others are powerful for of fense and defense. On account of the mating instincts, many higher animals have gained pe culiar methods of appealing to the eye or ear or smell of other members of the species.