190 



ZOOLOGY. 



consequently, the more organs there are endowed with 

 peculiar or specific kinds of activity, differing from each 

 other, the more numerous will be the number of dissimilar 

 parts in the animal economy ; and the complication more 

 or less great in the acts and faculties of animals must 

 proceed, pari passu, with the natural complication of their 

 organization. 



347. Thus in those animals in which the faculties are the 

 most limited, and in which life exhibits itself in its simplest 

 form, the body presents everywhere the same structure. 

 There is in fact a seeming identity of organization throughout. 

 Every part of the body performs the same functions as the 



neighbouring parts. If di- 

 vided into segments, "each 

 part lives, and becomes an 

 independent animal as com- 

 plete as that from which it 

 was violently separated. 



The fresh -water polyp or 

 hydra is an animal of this 

 kind. By mutilation it is 

 multiplied instead of being 

 destroyed. We owe the dis- 

 covery of these curious facts 

 to Trembley, a Swiss natu- 

 ralist of the last century. 



The simplicity of the orga- 

 nization of these animals can 

 only be demonstrated by the 

 microscope, under which the 

 substance of their body ap- 



Fig.Ml.-Hydra; Fresh-Water P e ? rs thljpughout identical; 

 Polyp.* it is composed of a gelatinous 



mass, enclosing fibrils and 



globules extremely minute. Now identity of structure would 

 imply identity of function, and the experiments of Trembley 

 proved the correctness of the inference. 



* In Figure 141 several polyps are represented as attached to water- 

 lentils, a; they consist of a single gelatinous tube, open at one of its 

 extremities, and furnished with a circle of filaments called tentacula, by 

 means of which they introduce into their bodies the food they require. One 

 of these polyps, b, carries on the sides of its body two small ones which spring 

 from it, and will soon be detached. In Fig. 4 (p. 19) may be seen one of these 

 animals magnified still more than the above, to show its internal confor- 

 mation. 



