22 MORPHOLOGY OF PROTOZOA 



form may occur. Thus in Trichomonas itself the undulating membrane 

 takes a slightly spiral course round the body, though its general tendency 

 is to be on the dorsal surface. Amongst the Ciliata this differentiation may 

 be carried to a high degree of complexity. In a few forms such as Prorodon 

 teres (Fig. 24) the cytostome is at the extreme anterior end of the body, and 

 the cilia pass in longitudinal rows from it to the posterior end. Though it 

 is possible in these cases to distinguish an anterior and posterior end, there 

 is actually no dorsal or ventral surface. In other forms the cytostome 

 has moved from its terminal position, and it at once becomes possible to 

 regard the surface on which the cvtostome is situated as the ventral one 



VJJ. 



m^f-^^ 



m 



Fig. 9. — Stylonychia mytilus : Side and Ventral Views ( x ca. 250). 

 (From Lang, 1901, Slightly Modified.) 



The side view shows the ciliate resting on a surface by means of the foot-like cirri formed by 

 fusion of groups of cilia. The dorsal cilia are few in number. There is a central contractile 

 vacuole with two excretory canals leading to it. 



The ventral view shows the macronucleus in division and two daughter micronuclei. The 

 V-shaped peristome is bordered on its outer edge by a row of membranes passing round the 

 anterior end of the ciliate and leading to the cytostome at the apex of the V. A row of cilia 

 borders the other edge of the peristome, within which is a longitudinal membrane. The 

 contractile vacuole and parts of the canal are seen as clear areas. 



(Fig. 14). In the majority of the free-living Ciliata there are definite 

 dorsal and ventral surfaces. These are most conspicuous in those forms 

 which lead a creeping mode of life, owing to the loss of cilia on the 

 dorsal surface, and the development of cirri and membranelles, through 

 the fusion of groups of cilia, on the ventral surface (Fig. 9). The cyto- 

 stome is on the ventral surface; it is not median in position, but displaced 

 to one side. In the case of attached forms such as Vorticella (Fig. 19) 



