HYDRA. 101 



but slow : for the hydra often pauses in the middle 

 of a step, as if deliberating whether it should pro- 

 ceed : so that the traversing a distance of seven or 

 eight inches is to these animals a very good day's 

 journey, even in summer. But a mode of travelling 

 rather more expeditious than this is occasionally 

 resorted to. It consists of a succession of somersets : 

 the hydra, while adhering firmly by the mouth, 

 detaches its foot, and, making it describe a semi- 

 circle, 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 

 continuing all the while elongated. 



By these and other manoeuvres these animals 

 contrive to walk with equal facility in any direction? 

 either on the bottom or sides of the vessel, or along 

 the stems of aquatic plants, to which they are most 

 frequently found attached. Their favourite position 

 is that of remaining suspended from the surface of 

 the water by means of the foot alone ; and this they 

 effect in the following manner. When the flat 

 surface of the foot is exposed 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 uncovered, and occupies the bottom of a 

 cup-shaped hollow in the fluid, thereby receiving a 

 degree of buoyancy 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 



VOL. I. M 



