THE FOOD OF ANIMALS. 47 
bon dioxide, water), some from the kidneys (water, urea, 
mainly), some from the skin (water, salines). (Respira¬ 
tion in part, Excretion.) 
The mechanism to accomplish all this in the lowest 
forms of life is exceedingly simple, a single cavity and 
surface performing all the functions. But in the major¬ 
ity of animals the apparatus is very complicated: there is 
a set of organs for the prehension of food; another, for 
digestion; a third, for absorption; a fourth, for distribu¬ 
tion ; and a fifth, for purification. 
CHAPTER VII. 
THE FOOD OF ANIMALS. 
The term food includes all substances which contribute 
to nutrition, whether they simply assist in the process, or 
are actually appropriated, and become tissue. With the 
food is usually combined more or less indigestible matter, 
which is separated in digestion. 
Food is derived from the mineral, vegetable, and animal 
kingdoms. Water and salt, for example, are inorganic. 
The former is the most abundant, and a very essential 
article of food. Most of the lower forms of aquatic life 
seem to live by drinking: their real nourishment, how¬ 
ever, is present in the water in'the form of fine particles. 
The Earthworm, some Beetles, and certain savage tribes 
of Men swallow earth; but this, likewise, is for the or¬ 
ganic matter which the earth contains. As no animal is 
produced immediately from inorganic matter, so no ani¬ 
mal can be sustained by it. 
Nutritious or tissue - forming food comes from the 
organic world, and is albuminous , as the lean meat of ani- 
