DIFFERENTIATION OF CILIA 117 



There is also a special differentiation of the cilia of tj^e 

 buccal groove (buc.gr!). On its left side is a single row of 

 very large and powerful cilia (A and c, m a.) 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 



FIG. 24. Oxytncha flava, killed and stained, showing the frag- 

 mentation of the nuclei. (After Gruber.) 



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. 



A third series of ciliated Infusoria must be just referred 

 to in concluding the present Lesson. We have seen how 



