PARAMECIUM _ 195 



the so-called undulating membrane. This structure {um. in 

 fig. 41) has the form of a fine transparent membrane, starting 

 like a frill near the anterior border of the mouth and passing 

 along its dorsal wall into the commencement of the narrow- 

 tube or gullet. 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 gullet 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 Metozoa. The ingestion of food by Paramecium may 

 easily be studied by adding finely-powdered carmine or sepia 

 to the water in which it is contained, 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 mouth and down the gullet by the action 

 of the cilia, their course being apparently guided by the un- 

 dulating membrane. At the extremity of the gullet a vacuole 

 is gradually formed in the endoplasm in which the solid par- 

 ticles are accumulated : the vacuole grows larger and larger 

 till it reaches the size represented in fig. 41, B. Then there 

 is a sort of gulp, a contraction of the surrounding protoplasm, 

 the vacuole is separated from the end of the gullet and passes 

 with its solid contents into the endoplasm, where it is at once 

 carried away by the cyclotic current, and a new vacuole forth- 

 with begins to form in its place. 



Just as a special aperture or mouth 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 

 and solid excretory substances to the exterior. This is 

 furnished by the so-called temporary anus, a spot situated a 

 little way behind the mouth opening on the ventral surface. 

 The anus is not a permanent aperture, or at least it is not 

 permanently open, and it can only be detected at the moment 

 when excreta are being discharged through it. Then it has 

 the appearance of a minute circular orifice placed upon a 

 small papillary projection of the body. But as soon as the 

 solid matter is voided the aperture and papilla vanish, and no 



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