DIFFERENCES BETWEEN ANIMALS AND PLANTS. 35 



Many of the lower animals resemble plants in their mode of gro^yth, 

 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 gen- 

 eral form and structure from that of one of the higher plants, it is 

 plain, from the facts vvhich 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 mor- 

 phological 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 inde- 

 pendent 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 phos- 

 phates, 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 pro- 

 tein 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 



