162 THIi MECHANICAL FUNCTIONS. 



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 hydro- 

 static 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 assistance 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 escape notice, 

 unless its previous situation were accurately- 

 observed. 



It might be asked by what power is this animal, 

 occupying so low a place in the scale of organiza- 

 tion, enabled to perform these actions? To this 

 question, however, no satisfactory answer has yet 

 been given. The substance of the hydra, when 

 examined by the microscope, appears to be nearly 

 homogeneous, except that a number of grains are 

 intermixed with the pulpy and gelatinous matter 



