CHAP. XXII.] CLASSIFICATION OF FOOD. 151 



In the lower Invertebrata, digestion becomes very simple. The intestine and 

 stomach become much reduced in size ; and in some the digestive apparatus 

 consists only of a simple bag, as in the fresh-water polyps, having the same 

 orifice of entrance and exit ; or, of a series of sacs or bags, with certain tubular 

 appendages, as in the Asterias, and in the Actinia ; in these latter animals, 

 one orifice answers equally for the introduction of the food, and for the dis- 

 charge of the superfluous matters. In Medusae, the oral aperture leads to a 

 capacious cavity or stomach, from which certain canals carry the nutritious 

 material into the different parts of the body, these canals being probably ana- 

 logous to the circulating systems of the higher animals. In some polyps, the 

 Bryozoa, as shewn by Dr. Arthur Farre, a portion of the stomach exhibits 

 great muscular power, and seems to perform the function of a gizzard. 



We shall find it convenient to examine the function of digestion 

 by tracing it through the various stages, as above enumerated, de- 

 scribing the mechanism of each of the subordinate processes, and 

 the change which each of them is capable of effecting in the food. 



Before we enter upon these points, we must make some remarks 

 upon the nature and quantity of the food suitable for the nourish- 

 ment of man. 



It has been already remarked at a former page (pp. 43,44, vol. i.), 

 that no food is suitable for the support of the human frame in a 

 healthy state, but that which contains the great staminal princi- 

 ples, which are the chief constituents of the body. And the same 

 remark applies with equal force to the carnivorous, and probably 

 to the herbivorous classes of animals. 



The food of the lower animals varies to a remarkable extent. 

 But nearly, if not entirely, throughout the series, organized mat- 

 ter, either vegetable or animal, forms the proper nutrient material. 

 It is probable that some of the lowest creatures enjoy the power of 

 assimilating inorganic matter, and thus become the instruments of 

 making the inorganic substances indirectly subservient to the 

 nutrition of the higher animals. 



The elements of nutrition for man, and the higher classes of ani- 

 mals, exist in the vegetable as well as in the animal kingdom. But 

 some animals are so constituted, that in a state of nature they sub- 

 sist only on the flesh of other animals ; while others live only upon 

 vegetable food. Some carnivorous animals, however, may, in a state 

 of domestication, be brought to eat vegetable food ; but it rarely, if 

 ever, happens that the herbivora can be taught to eat animal food. 

 Man is, by nature, a truly omnivorous animal ; and a certain ad- 

 mixture of animal and vegetable food is known, by experience, to 

 be that which is most conducive to his healthy nourishment. 



The classification of food which Dr. Prout has adopted, appears 



