04 THE VITAL FUNCTIONS. 



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 concentration of 

 organs and the union of many oftices in a single 

 organ, till we arrive in the very lowest orders, at 

 little more than a simple digestive cavity, perform- 

 ing 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, whether internal or external, 

 the muscular and nervous systems, the glands, 

 vessels, and organs of sense, have one after another 

 disappeared, we still continue to find the diges- 

 tive 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- 

 trasted with that of vegetables. It is a distinctive 

 criterion that applies even to the lowest orders of 

 zoophytes, which, in other respects, are so nearly 

 allied to plants. It extends to all insects, how- 

 ever diminutive ; and even to the minutest of the 

 microscopic animalcules.! 



The mode in which the food is received into the 



* See the first chapter of this volume, p. 9. 



t In some species of animals belonging to the tribe of Medusae, 

 as the Eudora, Berenice, Orijthia, Favonia, Lymnoria, and 

 Gcryonia, and also in the Liyula, among the Entozoa, no central 



