iiS PAKAMCECIUM, STVLONVCIIIA, OXYTRICIIA i.kss. 



less obvious : moreover, it has two mega nuclei {nu) and 

 only one contractile vacuole {c vac) 



But it is in the character of its cilia that Stylonychia 

 is most markedly distinguished from Paramcecium : these 

 structures, instead of being all alike both in form and size, 

 are modified in a very extraordinary way. 



On the dorsal surface the cilia are represented only by 

 very minute processes of the cuticle (c, d. a) set in longi- 

 tudinal grooves and exhibiting little movement. It seems 

 probable that these are to be looked upon as vestigial or 

 rudimentary cilia, />., as the representatives of cilia which 

 were of the ordinary character in the ancestors of Stylo- 

 nychia, but which have undergone partial atrophy^ or 

 diminution beyond the limits of usefulness, in correspond- 

 ence with the needs of an animalcule which has taken to 

 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. ci.), or like flattened rods frayed at 

 their ends (/. 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 single- 

 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 by means 

 of 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 9) ; in ciliary movement a flexion of 

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



