ii6 PARAMOECIUM, STYLONYCHIA, AND OXYTRICHA 



creeping on its ventral surface, instead of swimming freely 

 and so using all its cilia equally. 



On the other hand, the cilia on the ventral surface have 

 undergone a corresponding enlargement or hypertrophy. 

 Near the anterior and posterior ends and about the middle are 

 three groups of cilia of comparatively immense size, shaped 

 either like hooks (h. '.), or like flattened rods frayed at 

 their ends ( p. ci. and B). All these structures neither vibrate 

 rhythmically like ordinary cilia nor perform lashing move- 

 ments like flagella, but move at the base only like one- 

 jointed legs. The movement is under the animal's control, 

 so that it is able to creep about by the aid of these hooks 

 and plates in much the same way as a caterpillar uses its 

 legs. 



Notice that we have here a third form of contractility : in 

 amoeboid movement there is an irregular flowing of the pro- 

 toplasm (pp. 4 and 10) ; in ciliary movement a flexion of 

 a protoplasmic filament from side to side (p. 33) ; while 

 in the present case we have sudden contractions taking place 

 at irregular intervals. The movements of these locomotor 

 hooks and plates is therefore very similar to the muscular 

 contraction to which the movements of the higher animals 

 are due : it cannot be said that definite muscles are present 

 in Stylonychia, but the protoplasm in certain regions of the 

 unicellular body is so modified as to be able to perform a 

 sudden contraction in a definite direction. The nature of 

 muscular contraction will be further discussed in the next 

 Lesson (see p. 128). 



The remainder of the ventral surface, with the exception 

 of the buccal groove is bare, but along each side of the 

 margin is a row of large vibratile cilia, of which three at 

 the posterior end are modified into long, stiff, bristle-like 

 processes (A, b. d ). 



