160 THE MECHANICAL FUNCTIONS. 



animals.* While observing some aquatic plants, 

 Mhich 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 vegetables : but further observation con- 

 vinced 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 

 always placed themselves on the side of the glass 

 next to the light ; and by watching their changes 

 of position, he discovered the mode in which they 

 efltect their progressive motions. 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 



75 76 



then again fixed, preparatory to a new step, which 

 it takes by detaching the head and projecting it 

 forwards as before (Fig. 7(j). 



The progress made by these successive efforts is 



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

 douce, a bras en forme de cornes. Par A. Trembley, 1744. 



