MOORING THREAD OF VORTICELLA. 37 



liar and perhaps most wonderful organ of the Vorti- 

 eella, is its stalk or mooring thread. This though 

 generally of such extreme tenuity as to be almost 

 invisible with ordinary microscopes, yet exhibits a 

 remarkable degree of strength and muscular activity 

 in its movements, which apparently are more volun- 

 tary than those of the cilia. Its action consists of 

 a sudden contraction from a straight to a spiral 

 form with the coils closely packed together, by 

 which the head or bell is jerked down almost into 

 contact with the foot of the stalk ; after a few seconds 

 the tension seems gradually relaxed, the coils are 

 slowly unwound, and the stalk straightens itself out. 

 This action takes place at irregular intervals, but it 

 is seldom that more than a minute elapses between 

 each contraction. It (the contraction) invariably 

 happens when the animal is touched or alarmed, and 

 is, consequently, very frequent when the water 

 swarms with many other swimming animalcula. 

 When it takes place the flower-bell generally closes 

 up into a little round ball, which opens out again 

 only when the stalk becomes fully extended. From 

 this we might almost infer that some animalcule, or 

 other morsel of food, had been seized and retained 

 within the cup ; moreover, that the contraction of the 

 stalk assisted in securing or disposing of the prey. 

 This, however, is uncertain. 



The motions of the Vorticella do not seem much 

 affected by the stalk losing hold of its attachment ; 



