HYDRA. 1 33 



which has contributed to throw great light on the natural 

 history of polypiferous animals.^ While observing some 

 a(juatic plants, which he had collected and put into water^ 

 his attention was called to the aj)pearance of filaments ad- 

 hering to them, wiiich he had first conceived to be parasitic 

 vegetables: but farther observation convinced him that they 

 were endowed with powers of spontaneous motion, and that 

 they preyed upon small insects: and he, therefore, could no 

 longer doubt their animal nature. He found that they al- 

 ways placed themselves on the side of the glass next to the 

 light; and by watching their changes of position, he disco- 

 vered the mode in which they effect their progressive mo- 

 tions. If the hydra be standing in the erect position, its 

 foot being applied to the bottom of the glass (Fig. 73, ^ it 

 slowly bends the body in the direction in which it intends 

 to advance till its head touches the vessel, as shown in Fig. 

 74. It then adheres to the surface by the mouth, or by one 

 or two of its tentacula, and, detaching the foot, bends the 

 body into a curve, at the same time slightly retracting it, 

 so that the foot is brought near the head (Fig. 75.) The 

 foot is then again fixed, preparatory to a new step, which it 

 takes by detaching the head and projecting it forwards as 

 before (Fig. 76.) 



75 



76 



The progress made by these successive efforts is but slow: 

 for the hydra often pauses in the midst of a step, as if de- 

 liberating whether it should proceed: so that the traversing 

 a distance of seven or eight inches is to these animals a very 

 good day's journey, even in summer. But a mode of tra- 

 velling rather more expeditious than this is occasionally re- 

 sorted to. It consists of a succession of somersets: the hy- 



* Memolres pour servir a rHistoIro d'lin genre de Polypes d'eau douce, 

 a bras en forme dc cornes. Par A. Tremblev, 1744. 



