56 THE VITAL FUNCTIONS. 



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 de- 

 scend in the scale, we find them more and more simplified, 

 by the concentration of organs, and the union of many of- 

 fices in a single organ, till we arrive, in the very lowest or- 

 ders, 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, ves- 

 sels, 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 peculiar cha- 

 racteristic of the animal system as contrasted 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, howe- 

 ver diminutive; and even to the minutest of the microscopic 

 animalcules.! 



The mode in which the food is received into the body is, 

 in general, very different in the two organized kingdoms of 

 nature. Plants receive their nourishment by a slow, but 



* See the first chapter of this volume, p. 23. 



f In some species of animals belonging 1 to the tribe of medusze, as the 

 Eudora, Berenice, Orythia, Favonia, Lymnoria, and Geryonia, no central 

 cavity corresponding to a stomach has been discovered: they appear, there- 

 fore, to constitute an exception to the general rule. See Peron, Annales de 

 Museum, xiv. 227 and 326. 



