XVI INTEODUCTION. 



imbibe their nutriment by means of absorbent 

 organs situated on the exterior of their bodies, 

 while the food of animals is first received into an 

 interna] digestive cavity, there to be subsequently 

 elaborated. And even the greater number of 

 those simple animal beings, in which no definite 

 internal cavity can be perceived, would seem to 

 have the power of forming one, as it were, extem- 

 poraneously, whenever its presence is called for to 

 supply the wants of the organism. 



Every animal, as a livir.g being, possesses a 

 definite form, which is itself the product of a 

 number of definite parts or organs. In the higher 

 animals, the number of these dissimilar parts 

 attains its maximum. In others, no such com- 

 plexity of structure is observable, and there are 

 not wanting some animals, whose organisation is 

 so exceedingly simple, that any differentiation of 

 the body into separate parts can vdih difficulty be 

 determined. There are also many animal forms 

 of comparatively humble position, in which the 

 entire fabric consists, not of a few distinct organs, 

 each performing its special function, but of nu- 

 merous parts similar to one another, and all ful- 

 filling the same purpose. Such organisms are 

 said to exhibit a tendency to a "vegetative (or 

 irrelative) repetition of similar parts," ^ an expres- 

 sion which will be found convenient in practice, 

 provided it be borne in mind that the "similar 

 parts " are not Histinct individuals. 



