34 



MORPHOLOGY OF PROTOZOA 



flagella, which enable them to move about in search of the macrogametes 

 (Fig. 337). 



As noted above, the cilia which characterize the tiliophora resemble 

 small flagella. They have a similar structure, and their axial fibres take 

 origin in minute granules situated in the ectoplasm. It seems reasonable 

 to suppose that the axial fibres and the basal granules of cilia are homo- 

 logous with the axonemes and blepharoplasts of flagella. A single ciliate 

 possesses a large number of cilia, which exhibit more regularity and 

 co-ordination in their movements than the flagella of one of the Mastigo- 

 phora. In some ciliates the body is covered uniformly with cilia, which, 

 however, are usually arranged in longitudinal 

 rows (Fig. 14). In other cases the cilia are 

 limited to special regions of the body. The 

 cilia may be fairly uniform in length, but fre- 

 quently those on the extremities of the body 

 and those which surround the cytostome are 

 slightly longer than the others. Cilia are often 

 continued into the cytopharynx. Sometimes, 

 as in Cyclidium and other forms, one posterior 

 cilium is much larger than the others, and forms 

 a kind of tail or caudal process which has very 

 much the same size and structure as a flagellum 

 (Fig. 500). Several adjacent cilia may fuse 

 together to form stout processes known as cirri. 

 These are seen typically on the ventral surface 

 of those ciliates (Hypotrichida) which lead a 

 creeping mode of existence (Fig. 9). They 

 function as supporting structures or legs. In 

 some cases, again, rows of cilia may unite to 

 form membranes. This occurs frequently in 

 the cytopharynx of certain ciliates, such as 

 Paramecium, Pleuionema, and others (Fig. 70). 

 These membranes, or membranelles as the smp.ll 

 ones are often named, are distinct from the undulating membranes 

 of Mastigophora (trypanosomes), which are thin ridges of ectoplasm, 

 and are not formed by the fusion of rows of cilia. The cilia on the 

 peristome region near the cytostome may differ little from those on other 

 parts of the body. On the other hand, they may be considerably 

 modified in character and arrangement. In many forms they are 

 arranged as a spiral to form the adoral zone of cilia, which are continuous 

 with those in the cytopharynx. The spiral may be a left-handed spiral 

 or a right-handed one. It may consist of only a single turn or part of one. 



FiG. 14. — BaJaritidiuni ento- 



ZOOn FROM THE KeOTUM 



OF THE Frog ( x 650). 

 (Original.) 



The longitudinal rows of cilia on 

 the surface of the body are 

 represented by dots. 



