DIFFUSED CIRCULATION. 231 



which systematic zoologists have founded their 

 great divisions of the animal kingdom, the ut- 

 most importance is attached to those derived 

 from differences of structure in the organs of cir- 

 culation. 



A comprehensive survey of the diiferent classes 

 of animals 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 orders ; 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 difter from those adopted in the ve- 

 getable kingdom, where, as we have already 

 seen, the nutritive fluids, instead of being con- 

 fined in vessels, appear to permeate the cellidar 

 tissue, and thus immediately supply the solids 

 with the materials they require ; for, in the 

 simpler kinds of Polypi, of Infusoria, of Medusee, 

 and of Entozoa, the nourishment which has been 

 prepared by the digestive 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 denominated nourishment by 



