44 Instinct and Intelligence 



may be floating about within its reach. Hydra 

 can move from one spot to another without 

 difficulty. These movements are effected as 

 follows : the animal's body and tentacles ex- 

 pand and bend over to one side ; as soon as the 

 tentacles touch the solid substance to which its 

 foot is attached they become fixed to it; they 

 then contract, the animal's foot relaxes its 

 grasp, and its body moves so that the Hydra 

 comes to stand on its tentacles; the body then 

 bends over until its foot reaches something to 

 which it can fix itself, the tentacles loosen their 

 hold, and the animal resumes its former upright 

 position. In other cases the foot of the Hydra 

 glides along the surface it is attached to until 

 it comes close to the tentacles, which then leave 

 their hold, with the result that the Hydra moves 

 forward. When mechanically stimulated, the 

 body of a Hydra contracts at the same time as 

 its tentacles, as if to offer a smaller surface 

 to the cause of irritation; if, however, the 

 mechanical stimulus is continued, the animal 

 relaxes its foothold and moves away slowly in 

 the manner above described. 



If a Hydra is kept without food for a week or 



