ANIMALS AND PLANTS. 123 



cess, previously to being absorbed; the food of plants is always pre- 

 sented to them already in a state of solution. Animals, as a rule, re- 

 ceive their food into a mouth leading to a distinct stomach, or into 

 some other internal permanent cavity, as in the Infusoria, or into a 

 temporary cavity only, as in the Amoeba and Actinophrys ; whilst 

 plants have no stomach or digestive cavity at all, but absorb their food 

 directly at some part of their surface. But even this obvious distinc- 

 tion is not universal, the lowest Protozoa, viz., the Gregarinida, not 

 possessing a stomach, or even a temporary digestive cavity. 



The chief point of distinction between the food of animals and plants, 

 relates to its source and chemical composition ; and this constitutes a 

 more positive ground of distinction between them. Animals require 

 food already prepared by some pre-existing organism ; that is to say, 

 food composed of certain proximate constituents named organic sub- 

 stances, because they are the result of chemical combinations at pres- 

 ent only known to occur in the living bodies of animals or plants. 

 Animals, therefore, feed upon either animal or vegetable matter, i. e., 

 on organic substances. Bat plants feed exclusively on inorganic sub- 

 stances, derived from the atmosphere, the water, and the soil. The 

 chemical composition of the organic food of animals, for they too re- 

 quire air, water, and certain mineral constituents, is highly complex, 

 consisting of the quinary substances known as albuminoid bodies, or 

 their immediate derivatives, besides the carbhydrates, starch, and 

 sugar, and the hydrocarbons, or fatty substances, all of which are re- 

 duced to a state of minute division, or solution, by special digestive 

 processes, and are then assimilated. These substances are obtained, 

 in the case of carnivorous animals, from other animals ; in the case 

 of the Herbivora, from plants ; in the case of the Omnivora, from 

 both sources; ultimately they are always derived from the vegetable 

 kingdom. On the other hand, the food of plants, in addition to the 

 necessary saline mineral constituents, consists of carbonic acid, water, 

 and ammonia, which binary chemical compounds they, by aid of their 

 vito-chernical processes, stimulated by light and heat, combine into the 

 carbhydrates, to form starch, sugar, gum, and woody fibre, the hydro- 

 carbonaceous oils, and the quaternary and quinary nitrogenous albu- 

 minoid bodies, such as thein, legumin, and gluten. Plants, therefore, 

 appear to have the power of forming, as the highest product of their 

 vital processes, from carbonic acid, water, ammonia, and sulphur, albu- 

 minoid matter a power not possessed by animals. Plants also directly 

 form the carbhydrates, and probably from these, the hydrocarbons ; 

 whilst if animals produce these bodies, it is supposed to be from the 

 decomposition of albuminoid substances. It has been noted that the 

 organic acids and bases formed in animals, are much fewer than those 

 met with in plants, and that but a small number are common to both 

 kingdoms. 



Lastly, the nature of the chemical processes which occur in the 

 economy of animals and plants, must necessarily differ, in accordance 

 with the difference in their respective food. In animals, the organic 

 constituents of the food once digested, absorbed, and assimilated, un- 

 dergo, under the influence of a certain temperature, many changes, 



