THE rOOD OF ANIMALS. 47 



bon dioxide, water), some from the kidneys (water, nrea, 

 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 YII. 



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. 

 Tiie Earthworm, some Beetles, and certain savage tribes 

 of Men swallow eartii ; but this, likewise, is for the or- 

 ganic jnatter 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- 



