134 THE MECHANICAL FUNCTIONS. 



dra, while adhering firmly by the mouth, detaches its foot, 

 and, making it describe a semicircle, throws it over its head, 

 and places it foremost in the line of progression. Having 

 attained this situation, the foot is then -fixed, and a similar 

 semi-revolution is performed by the head, the body conti- 

 nuing all the while elongated. 



By these and other manoeuvres these animals contrive to 

 walk with equal facility in any direction, either on the bot- 

 tom or sides of the vessel, or along the stems of aquatic 

 plants, to which they are most frequently found attached. 

 The position in which they appear to take most delight, is 

 that of remaining suspended from the surface of the water 

 by means of the foot alone: and this they effect in the fol- 

 io win o- manner. When the flat surface of the foot is ex- 

 posed for a short time to the air, above the surface of the 

 water, it becomes dry, and in this state exerts a repulsive 

 action on the liquid: so that when dragged below the level 

 of the surface by the weight of the body it still remains un- 

 covered, and occupies the bottom of a cup-shaped hollow 

 in the fluid, thereby receiving a degree of buoyancy suffi- 

 cient 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 gen- 

 tly 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 hy- 

 drostatic power will be destroyed, and the animal will im- 

 mediately 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 an- 

 chors, to enable them to retain their positions in security, 

 however violently the water may be agitated. Great use is 



