96 MANUAL OF PHYSIOLOGY. 



of the protoplasm can suddenly and forcibly contract, so as to 

 completely alter its shape, and change the bell into a rounded 

 mass. This spasm of the body is commonly associated with a 

 wonderfully rapid contraction of the stalk. This stalk consists 

 of a delicate transparent sheath, in the centre of which is a thin 

 thread of pale protoplasm. The rapid contraction of the proto- 

 plasm of the stalk and the spasm of the bell occur on the appli- 

 cation of the least mechanical excitation, such as a touch to the 

 cover glass. Here in a single cell we have certain portions set 

 apart for special purposes, most of which are the same as in para- 

 moecia. But the animal being attached requires a special way of 

 escaping from its enemies, and hence we find it endowed with 

 three special forms of motion. Besides the ciliary and stream- 

 ing protoplasmic motion, its body can spasmodically change its 

 shape, and the stalk contracts with a velocity comparable with 

 that of the most specially modified contractile tissue (muscle) of 

 the higher animals, by means of which their rapid and varied 

 movements are carried out. 



