m Organized Beings. 101 



separated. I put aside for the present that series of changes 

 occurring alike in the plant and the animal, which have for 

 their object the continuance of the race, not the maintenance 

 of the individual ; these will be a subject for after consideration. 

 The organic functions being common to both kingdoms, it be- 

 comes a most interesting topic of inquiry how far the organs 

 which perform them have the same elementary structure in 

 each, how far the changes produced by them are sin;ii;!r, and 

 how far analogy can',be traced in their gradual special .'.ation. 



As all the changes which are essential to the existence of a 

 living organism may be regarded as consisting in the assimilation 

 of matter from without, and the libei'ation of excrementitious 

 matter from within, so the two functions of absorption and ex- 

 cretion may be regarded as comprehending the sum of the acts 

 by which these changes are produced. In the lowest plants 

 and animals we find no provision for any more complicated pro- 

 cesses. In the Algas, for example, the whole surface is absorb- 

 ent ; no part more than another can be regarded as peculiarly 

 exercising this function ; every cell derives from the fluid in 

 contact with it, or from the surcharged cells in its immediate 

 neighbourhood, the fluid essential to its existence. In like man- 

 ner we might advert to the structure of the gemmules of the 

 Porifera and Polypifera as furnishing an example of a similar 

 mode of nutrition in the animal kingdom ; but as these beings 

 are mere embryos, it is not perhaps fair to adduce them in il- 

 lustration. Wo very early find in the animal kingdom a tend- 

 ency to specialization of the organs of absorption, by the ap- 

 propriation of a continuation of tlie external surface for the pur- 

 pose. In the common hydra, for example, we may regard the 

 animal as entirely composed of a stomach and its appendages ; 

 and this stomach may be regarded as simply a reflection of the 

 tegumentary membrane inxcards.. as the experiments of Trem- 

 bley sufficiently prove, by shewing the mutual convertibility cf 

 these two surfaces. We may then express the form of the ab- 

 sorbent portion of the general surface by such a sketch as the 

 following (Fig. 1). Now, although we have here a 

 decided internal stomach, we still have the tissues de- 

 riving their nutriment by immediate absorption, 

 partly from the fluid within the bag, and partly from 

 that on the exterior, as the experiments before allud- 



