178 THE MECHANICAL FUNCTIONS. 



tables : but farther observation convinced him 

 that they were endowed with powers of spon- 

 taneous 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 w atching their 

 changes of position, he discovered the mode in 

 which they eflbct 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 direc- 

 tion 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 ncAv step, which it takes 

 by detaching the head and projecting it forwards 

 as before (Fig. 70). 



The progress made by these successive efforts 

 is but slow : for the hydra often pauses in the 

 midst of a step, as if deliberating whether it 



