Comparative Physiology. 469 



the ingestion of solid aliment by the higher animals does not 

 contradict this principle, when the character of the nutritive 

 organs are examined. Whilst the roots of vegetables ramify- 

 through the soil in pursuit of nutriment, animals, whose locomo- 

 tive powers are necessary to search after food, may be said to 

 carry their soil about with them, for their absorbents are dis- 

 tributed on the walls of the digestive cavity, just as those of 

 plants are prolonged into the earth. This cavity is in all in- 

 stances formed by a reflection of the external surface, and in the 

 lower classes is merely a bag with one opening, which may be 

 regarded as all stomach. The food is acted on mechanically by 

 the motion of the walls, and chemically by the secretions poured 

 from their surface separating the nutritious parts, and reducing 

 them to a fluid form. The process of animal digestion has no- 

 thing to do with organising or vitalising the materials submitted 

 to it. Some physiologists have regarded the possession of a di- 

 gestive cavity as the most prominent characteristic of animals, 

 but there is no doubt that many of the lower classes, during a part 

 of their existence at least, are nourished by absorption from the 

 exterior surface alone. The earth has been justly spoken of as 

 the common stomach of vegetables ; but the pitchers and traps 

 of some plants approach the nature of a stomach, the insects ap- 

 peai-ing to serve as nutriment to them. In all cases, however, 

 where previously organised matter influences the growth of 

 plants, it is whilst in a decomposing state, and separated into 

 its ultimate elements or very simple combinations of them. In 

 animal digestion, on the contrary, the proximate principles of the 

 food appear immediately subservient to the formation of others 

 of a higher order, any tendency to decomposition being checked 

 by the antiseptic qualities of the gastric fluid. The earth-worm 

 and some beetles, which swallow earth, do so only to obtain the 

 remains of the organised matter mixed with it. All the tribes of 

 plants have their peculiar animals and insects which feed on 

 them, and keep them in check; and all the classes of animals have 

 their carnivorous tribes, adapted to restrain the too rapid increase 

 of the vegetable feeders. Those animals which obtain their food 

 with most facility seem least able to endure privation. 



" It has been stated that all alimentary materials before being 

 introduced into the living system must be presented to it in a 

 fluid form. The changes involved in its passage through the 

 membrane, or external integument, constitute the function of 

 absorption. The transmission always takes place through some 

 tissue of a membranous character, and never through open 

 mouths of vessels. The skin of the higher animals, and the 

 cuticle of plants, participate more or less in the function of ab- 

 sorption, even where a special absorbent system is provided ; in 

 the inferior tribes the external integument is its sole medium. 



