THE CILIATA 223 



under a magnification of 1000 diameters is given in fig. 47, B. 

 It commences as a wide funnel-like cavity at the posterior end 

 of the peristomial groove. The cuticle is reflected so as to line 

 the cavity, and the cilia are also continued into it. The mouth 

 narrows somewhat rapidly to form a narrow ciliated tube or 

 cytopharynx which is directed at first upward, then turns 

 downward with a slight spiral twist and ends in the endoplasm 

 of the posterior third of the body. The entrance to the 

 cytopharynx is guarded by the so-called undulating membrane. 

 This structure (um. in fig. 47) has the form of a fine trans- 

 parent membrane, starting like a frill near the anterior border 

 of the mouth and passing along its dorsal wall into the com- 

 mencement of the narrow cytopharynx. The free edge of 

 the frill hangs down into the cavity of the mouth, and its other 

 edge is obliquely inserted in the dorsal wall of the mouth cavity. 

 The membrane vibrates rapidly with an undulating movement, 

 and is evidently formed from a row of fused cilia, for its free 

 edge is often frayed in the manner shown in the drawing. The 

 inner end of the cytopharynx is furnished with sparse and 

 rather stout cilia, which move with a relatively slow undulating 

 movement, recalling the ciliary action of the so-called flame-cells 

 in the excretory organs of certain Metazoa. The ingestion of 

 food by Paramecium may easily be studied by adding finely 

 powdered carmine or sepia to the water in which it is con- 

 tained, or very frequently, if the specimen under observation is 

 imprisoned in freshly-made gum-arabic, it will swallow the gum. 

 Particles may be seen to be driven into the cytostome and down 

 the cytopharynx by the action of the cilia, their course being 

 apparently guided by the undulating membrane. At the 

 extremity of the cytopharynx a vacuole is gradually formed in 

 the endoplasm in which the solid particles are accumulated : 

 the vacuole grows larger and larger till it reaches the size 

 represented in fig. 47, B. Then there is a sort of gulp, a con- 

 traction of the surrounding protoplasm, the vacuole is separated 

 from the end of the cytopharynx and passes with its solid con- 

 tents into the endoplasm, where it is at once carried away by 

 the cyclotic current, and a new vacuole forthwith begins to 

 form in its place. 



Just as a special aperture or cytostome is required to admit 

 of the entrance of solid food into the endoplasm, so a special 

 exit is required for the passage of insoluble remnants of food 



