180 THE MECHANICAL FUNCTIONS. 



in the fluid, thereby receiving a degree of buoy- 

 ancy sufficient to suspend it at the surface. The 

 principle is the same as that by which a dry 

 needle is supported on water in the boat-like 

 hollow which is formed by the cohesive force of 

 the liquid, if care be taken to lay the needle 

 down very gently on the surface. If, while the 

 hydra is floating in this manner, suspended by 

 the extremity of the foot, a drop of water be 

 made to fall upon that part, so as to wet it, this 

 hydrostatic power will be destroyed, and the 

 animal will immediately sink to the bottom. 



While in this state of suspension from the 

 surface, the hydra is capable of performing 

 several curious evolutions, and with the assist- 

 ance of the tentacula, by which it lays hold of 

 objects within its reach, is able to cross over 

 from one side of the vessel to the other. It does 

 not appear that these animals ever employ the 

 tentacula as instruments for swimming ; but they 

 frequently use them as cables, or anchors, to 

 enable them to retain their positions in security, 

 however violently the water may be agitated. 

 Great use is also made of the tentacula as organs 

 of prehension for seizing and detaining their 

 living prey, and for conveying it to the mouth, 

 where it is quickly swallowed. On the other 

 hand, when alarmed, or exposed to irritation, 

 the hydra suddenly shrinks, by the gradual 

 contraction of all the tentacula, and of the body 

 also, into a small globule, which might easily 



