MORPHOLOGY AND PHYSIOLOGY 29 



tion results in bending of the cilium and its relaxation in straight- 

 ening of it. The cilia are generally arranged in longitudinal, 

 oblique or spiral rows. In young Suctoria, the ciliary rows are 

 transverse (Fig. 173, c, d). Although cilia are primarily the cell- 

 organs of locomotion, they serve directly or indirectly for food- 

 capture also. 



In some ciliates, there are much stouter and considerably 

 longer cytoplasmic projections, known as cirri (Fig. 10). They 

 may occur with the cilia or completely replace them. A cirrus 

 is composed of a number of cilia whose roots produce a basal 

 plate, and from this thick base it tapers gradually to a point. 

 In some cases, the end of a cirrus may show two or more 

 branches (Fig. 167). The cirri usually are confined to the ventral 

 surface of the organism and are called frontal, marginal, ventral, 

 anal, and caudal cirri according to their location (Fig. 10). 

 Unlike cilia, cirri may move in any direction, so that organisms 

 possessing them show various ways of locomotion. 



In all ciliates except the Holotrichida, there occurs an 

 adoral zone on the left margin of the peristome, which consists 

 of a number of triangular or plate-like membranellae (Fig. 9). 

 Each membranella is composed of numerous cilia which are 

 completely fused into one mass. The adoral zone seems to serve 

 primarily for bringing food particles to the cytostome. Still 

 more common is the undulating membrane which is composed 

 of one or more rows of longitudinally placed cilia (Fig. 10). It 

 is, therefore, different in structure from the so-called undulating 

 membrane found in some Mastigophora mentioned above. The 

 undulating membrane in the cytopharynx of Paramecium and 

 that in the peristome of Pleuronema (Fig. 158) are typical 

 examples. 



In Suctoria, except one genus, cilia are present only during 

 developmental stages, and as the organisms become mature, 

 tentacles are formed. The latter are either suctorial or prehen- 

 sile in function and are often very long. Each contains a highly 

 contractile axial filament. In a few instances the tentacle is 

 tubular, and this type is interpreted by Collin as possibly de- 

 rived from a cytostome and cytopharynx of the ciliate (Fig. 11). 



Although the vast majority of Protozoa, possess only one of 

 the three cell-organs of locomotion mentioned above, a proto- 



