ANIMAL and ANIMAL KINGDOM. According to a very old classification, all bodies are divided into three kingdoms—the mineral, the vegetable, and the animal. Animals and vegetables are again classed together as organic, in opposition to minerals, which are inorganic. Mineral bodies are masses of matter without internal movement, increas ing by additions from without, having, with the exception of crystals, no determinate form or size, homogeneous throughout, and without relation of one part to another. Animals and plants, on the contrary, exist as individual beings, consisting of various organs. Their existence has a beginning and an end, and at their death they are replaced by other similar beings developed out of them.
The distinction between animals and plants strikes us at once in the higher classes; but among the lower organisms there are beings which have been at times classed among animals, at times among plants. The marks of anima]ity which, with very few exceptions, have as yet been found to exist in all animals, are spontaneous motion, the existence of a special digesting apparatus (it may be only a mouth and a stomach), and sensation by means of nerves. The prevalence of nitrogen as a chemical ingredient is another general characteristic of animals, while carbon prevails in plants. Most am mals, though not :Al, possess the faculty of /ow-motion (q.v.); it is wanting in some, as the oyster and polype. This locomotion is generally effected by appropriate organs, which are very different in the different classes of animals, as legs, wings, fins, suckers, cilia, etc., sometimes merely by muscular dilatations and contractions. In the higher animals, it is connected with a special system of bones and muscles, which becomes less and less prominent as we descend in the scale, and at last disappears.
Nutrition is effected by swallowing and digesting organic matter by means of a mouth, stomach, and intestinal canal. A part of the food—the chyle, namely, which results from digestion—is taken up by a system of vessels into the body of the animal, and thrown into the blood, into which, under the action of the air in the lungs or gills, it is converted; the other part is excreted by a second orifice, except in some of the lowest forms where the mouth forms the exit. For keeping up a circulation of the
blood, which must be brought to all parts of the body for the purpose of nourishment, there is provided a system of blood-vessels and, in the higher classes, a heart. Sue IIEAirr. Nutrition may, to some extent, take place also by absorption from the external surface; hut this is not considerable except, perhaps, among the lowest classes of ani mals. The substances that serve for the nutrition of an animal are either vegetable or animal, and the mouth and other organs are adapted accordingly. The number of om nivorous animals is small, and among these man has the greatest latitude of choice.
Propagation or reproduction takes place in a great variety of ways; among the lowest forms, by division, gemmation or budding, and cell-germs; am mg the more per fect, by generation between two individuals of different sex. Of the two sexes, the male is generally distinguished by superior size and strength, more brilliant coloring, larger appendages, and often stronger voice. Besides male and female, there are among some animals (bees and ants) neuters. In some of the lower kinds, the individuals are hermaphrodite. See REPRODUCTION.
All animals develop gradually, and most of them go through one or more changes of form or metamorphoses. This is most marked among insects which go through the four stages of egg, larva, pupa, and perfect insect. The class of reptiles with naked skins also go through changes, though less striking. In the higher animals, these transitions through a series of forms take place in the ovum, or before birth. In some cases the embryo comes to maturity after the exclusion of the ovum (birds and amphibia); in others (mammalia), within the body of the mother: animals of the last kind are called viviparous. The reproduction of some intestinal worms is peculiar; the egg of the mother-animal produces a sexless creature—a nurse—the eggs laid by which reproduce the original animal. A somewhat similar peculiarity is observed in some insects, as aphides. See APHIS.