RECEPTACLES OF FOOD. 71 



channels of circulation, not only to the respi- 

 ratory organs, where its elaboration is completed 

 by the influence of atmospheric air, but also to 

 all other parts of the system, where such a supply 

 is required for their maintenance in the living 

 state. The objects of these subsequent functions, 

 many of which are peculiar to animal life, have 

 already been detailed.* 



This subdivision of the assimilatory processes 

 occurs only in the higher classes of animals ; for 

 in proportion as we descend in the scale, we 

 find them more and more simplified, by the con- 

 centration of organs, and the union of many offices 

 in a single organ, till we arrive, in the very lowest 

 orders, at little more than a simple digestive 

 cavity, performing at once the functions of the 

 stomach and of the heart ; without any distinct 

 circulation of nutrient juices, without vessels, — 

 nay without any apparent blood. Long after 

 all the other organs, such as the skeleton, whe- 

 ther internal or external, the muscular and ner- 

 vous systems, the glands, vessels, and organs of 

 sense, have one after another disappeared, we 

 still continue to find the digestive cavity retained, 

 as if it constituted the most important, and only 

 indispensable organ of the whole system. 



The possession of a stomach, then, is the pecu- 

 liar characteristic of the animal system, as con- 



* See the first chapter of this volume, p. 11. 



