HYDRA. 179 



should proceed ; 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 con- 

 sists 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 vralk vrith 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. 

 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 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 



