x DIFFERENTIATION OF CILIA 119 



contractions 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. 130). 



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. a). 



There is also a special differentiation of the cilia of the 

 buccal groove (buc.gr.']. On its left side is a single row of 

 very large and powerful cilia (A and c, m. cf] which are the 

 chief organs for causing the food-current as well as the 

 main swimming-organs : each has the form of a triangular 

 fan-like plate (c, m. a). On the right side of the buccal 

 groove is a row of smaller but still large cilia of the ordinary 

 form, and in the interior of the gullet a row of extremely 

 delicate cilia which aid in forcing particles of food down the 

 gullet into the medulla. 



In Stylonychia and allied genera intermediate forms are 

 found between these peculiar hooks, plates, bristles, and 

 fans, and ordinary cilia ; from which we may conclude that 

 these diverse appendages are to be looked upon as highly 

 modified or differentiated cilia. Probably they have been 

 evolved in the course of time from ordinary cilia, and on 

 the principle that the more complicated or specialized 

 organisms are descended from simpler or more generalized 

 forms (see Lesson XIII.), we may consider Stylonychia as 

 the highly-specialized descendant of some uniformly-ciliated 

 progenitor. 



