Many of the lower animals resemble plants in their mode of growth, as well as in simplicity of structure; the colonies or clusters of the compound hydroids, and the coral-making polyps, are very plant-like, and, as they also lack the power of locomotion, their animal nature was for a long time disputed, and they were classified as plants.
However much the body of one of the higher animals differs in general form and structure from that of one of the higher plants, it is plain, from the facts which we have pointed out, that the two groups cannot be absolutely and arbitrarily separated upon this basis, although the general value of the distinction is obvious.
Histology furnishes another difference which is quite general but not universal. There are well-marked and pretty constant contrasts between the cells and tissues of the one group and those of the other, and these contrasts furnish what is, perhaps, the most constant morphological distinction. The constituent cells of the tissues of the plant retain their original form and individuality, while, in the animal, they undergo the greatest modifications, and their individuality is usually entirely lost. A vegetable tissue or organ may easily be shown to be made up of a mass of nearly similar cells, each of which is independent and sharply defined; while the tissues of animals present the greatest differences in structure and appearance, and sharply-defined individual cells are seldom to be seen. The cause of this difference in the appearance of the tissues is a difference in the cells themselves. The protoplasmic contents of the vegetable cell are inclosed by a thick, strong outer membrane or cellulose wall; while the outer surface of the animal cell is usually only a little more dense than the protoplasmic contents, and does not usually form a distinct cell-wall. The vegetable cell may, however, be destitute of the cellulose wall; and, on the other hand, many animal tissues—cartilage, for instance—resemble the tissues of plants in being made up of independent cells, each of which has an outer layer.
The most universal and characteristic difference between animals and plants is physiological, and relates to the nature of the food and the character of the nutritive process. From comparatively simple inorganic substances, such as water, carbonic acid, and ammonia, the plant is able to build up the highly-complex protein compounds which are so characteristic of living beings. The animal feeds, in part, upon inorganic substances, such as water, and certain carbonates and phosphates, but it derives all its protein from plants, either directly, or, as in the case of the carnivorous animals, indirectly, through the aid of vegetable-feeding animals. In the body of the animal the complex protein compounds are broken down into simpler substances, and the energy thus set free is converted into the various manifestations of "vital force." The animal organism is thus a consumer of protein and a liberator of force. The vital activities of the plant depend, like those of the animal, upon the liberation of energy by the breaking