HYDRA. 177 



of being bent in all directions. In this way, 

 they can quickly surround and grasp any small 

 object which they may happen to touch ; and 

 whenever irritated they instantly retract, so as 

 hardly to be visible without the aid of a mag- 

 nifier. Each tentaculum may be moved inde- 

 pendently of the rest, at the pleasure of the 

 animal. The remainder of the body tapers 

 gradually from the head to the other extremity, 

 becoming very slender, and having at its termi- 

 nation a flat surface, which has been termed the 

 foot ; for although every portion of the surface 

 has the power of adhering to the bodies to which 

 it is applied, it is principally by this extremity 

 that the animal chooses to attach itself to the 

 sides or bottom of the vessel in which it is kept. 

 No trace of the existence of cilia can be met with 

 on any part of the surface of these animals. 



It is to Mr. Trembley of Geneva that we are 

 indebted for the discovery of this singular 

 animal, the examination of which has contri- 

 buted to throw great light on the natural history 

 of polypiferous animals.* While observing some 

 aquatic plants, which he had collected and 

 put into water, his attention was called to the 

 appearance of filaments adhering to them, 

 which he at first conceived to be parasitic vege- 



* Memoires pour servir a I'Histoire d'un genre de Polypes 

 d'eau douce, a bras eu forme de cornes. Par A. Trembley, 1744. 

 VOL. I. N 



