22 ON POLYPS. 



ther appearances of organization : there is no trace of muscular 

 fibre or of nervous substance, not the slightest indication of vessels 

 of any kind, nor any apparatus destined to the function of repro- 

 duction ; such is the hydra, offering in every particular a good 

 example of the acrite type of structure. 



The young naturalist would scarcely be prepared to see an 

 animal of this description waging continual war with creatures 

 much more perfectly organized than itself; endowed with consi- 

 derable capability of locomotion ; possessed not only of a refined 

 sense of touch, but able to appreciate the presence, and seek the 

 influence of light ; and exhibiting moreover a tenacity of life and 

 power of reproduction almost beyond belief : a little observation, 

 however, will convince him that it possesses all these attributes, 

 and enable him to share in some degree the astonishment with 

 which Trembley, their enthusiastic discoverer, first witnessed and 

 described them.* 



(27.) The hydra is not like most other polyps, fixed and station- 

 ary; but can roam about and change its situation according to circum- 

 stances. Its usual mode of progression is by creeping along the 

 stems of aquatic plants, or upon the sides of the glass in which it 

 is confined : attaching first the little tubercle at its posterior ex- 

 tremity to the surface upon which it moves, it slowly inflects its 

 body (fig. 4, 3), and fixing its oral tentacles, moves along in the 

 manner of a leech, by a succession of similar actions. This method 

 of advancing is, from the small size of the animal, necessarily slow ; 

 and a march of a couple of inches will require several hours for its 

 performance : but, when arrived at the surface of the water, it adopts 

 a more speedy course ; suspending itself by the tail as by a minute 

 float, and hanging with its mouth downwards, it rows itself about 

 with its tentacles, or, wafted by the wind, can travel to a consider- 

 able distance without effort. 



(28.) When left free, the hydrse are found to select positions most 

 exposed to the influence of light, assembling at the surface of the 

 ponds which they inhabit, or seeking that side of the glass in which 

 they are confined, that is most strongly illuminated. That they 

 are able to appreciate the presence of light is therefore indubitable ; 

 yet with what organs do they perceive it ? We are driven to the 

 supposition that, in this case, the sense of touch supplies to a certain 

 extent the want of other senses, and that the hydrse are able, as 



* Trembley, Memoires pour servir a 1'Histoire des Polypes d'eau douce. Leyde, 

 1744. 



