168 THE VITAL FUNCTIONS. 



must influence every other function of the economy, and 

 must, therefore, constitute an essential element in deter- 

 mining the physiological condition of the animal. We fmd, 

 accordingly, that among the characters on which systematic 

 zoologists have founded their great divisions of the animal 

 kingdom, the utmost importance is attached to those de- 

 rived from differences of structure in the organs of circula- 

 tion. 



A comprehensive survey of the different classes of ani- 

 mals with reference to this function, enables us to discern 

 the existence of a regular gradation of organs, increasing in 

 complexity as we ascend from the lower to the higher or- 

 ders; and showing that here, as in other departments of the 

 economy of nature, no change is made abruptly, but always 

 by slow and successive steps. In the very lowest tribes of 

 Zoophytes, the modes by which nutrition is accomplished 

 can scarcely be perceived to differ from those adopted in the 

 vegetable kingdom, where, as we have already seen, the nu- 

 tritive fluids, instead of being confined in vessels, appear to 

 permeate the cellular tissue, and thus immediately supply 

 the solids with the materials they require; for, in the sim- 

 pler kinds of Polypi, of Infusoria, of JVIedusee, and of Ento- 

 zoa, tlie nourishment which has been prepared by the di- 

 gestive cavities is apparently imbibed by the solids, after 

 having transuded through the sides of these organs, and 

 without its being previously collected into other, and more 

 general cavities. This mode of nutrition, suited only to the 

 torpid and half vegetative nature of zoophytes, has been de- 

 nominated nourishment by imbibition, in contradistinction 

 to that by circulation^ a term, which, as we have seen, im- 

 plies, not merely a system of canals, such as those existing 

 in Medusae, where there is no evidence of the fluids really 

 circulating, but an arrangement of ramified vessels, composed 

 of membranous coats, through which the nutrient fluid moves 

 in a continued circuit. 



The distinction whicli has thus been drawn, however, is 

 one on which wc should be careful not to place undue reli- 



