ELEMENTARY ORGANIC STRUCTURE. 601 



greatly elongate the body, and also contract itself into a 

 small globular mass, as in the uppermost polype in Jig. 2.* 



The most extraordinary vital endowment possessed by 

 these freshwater polypes, is that of the reproduction of lost 

 parts to an almost unlimited extent ; even to the formation of 

 several perfect animals from the separated pieces of a 

 single individual. If the body is split in half, each portion 

 grows into a complete Hydra, as is shown in this drawing 

 (Jig. 9) ; and as if there were no limits to its transforma- 

 tions, the creature may be turned inside out, and that which 

 was the surface of the stomach will become the epidermis, 

 and the outer skin form the lining of the new stomach, and 

 carry on the process of digestion ! f 



9. Elementary organic structure ; Cells. — The 

 interpretation of these phenomena is to be found in the 

 peculiar organization of the Hydra, its entire structure 

 being nothing more than an aggregation of cells. A ver- 

 tical section of a Polype (Lign. 132, Jig. o,) shows the 

 internal cavity or digestive sac, the relative thickness of 

 the substance of the body, and the manner in which the 

 arms are formed by a prolongation of the upper part into 

 hollow processes. This animal is, in fact, a simple sac or 

 pouch formed of a congeries of cells, for the reception and 

 assimilation of food. The cells lining the stomach select 

 and absorb the nutritious particles, and the tube then 

 spontaneously contracts and casts out the residue of diges- 

 tion. This organization is analogous to that of the simplest 

 condition of the vegetable kingdom, the Cellulosce : for even 

 the large Furf, or sea-weeds, consist only of cells. The 

 freshwater Confer vce are merely jointed films composed of 

 cells, containing granules or lesser cells. A cell bursts, 

 * See my " Thoughts on Animalcules/' PL I. for coloured figures of 

 the Hydrae. 



j B£ Tremhley of Geneva, in 17-10. first observed this wonderful pro- 

 perty of the Hydra. See his charming work, " Memoires pour servir 

 a l'Histoire dun Genre de Polypes d"Eau douce."' A Leide, 1744. 



