PARAMECIUM 5 



tinctness with the movements of the animal, and 

 are probably due to longitudinal wrinkling of the 

 inner surface of the ectosarc. 



a. The cuticle is the delicate superficial and stiffer 



layer, serving as a protective covering for the 

 underlying protoplasm, of which it is the dif- 

 ferentiated external layer. 



b. The cilia are very numerous delicate vibratile fila- 



ments arising from the layer of ectosarc imme- 

 diately beneath the cuticle, through which latter 

 they project. They are of uniform size over the 

 entire surface, and extend into the oral groove. 

 Their rapid movements serve both for locomotion 

 and for the ingestion of food. 



c. The trichocysts are minute oval sacs arranged side 



by side in the deeper part of the ectosarc, per- 

 pendicular to the surface, and in such numbers 

 as to form an obvious layer. When the animal 

 is irritated a stiffish thread can be shot out from 

 each of these trichocysts, and project on the sur- 

 face beyond the cilia. They are protective and 

 offensive weapons. 



d. Two ptosating vacuoles are situated in the sub- 



stance of the ectosarc of the dorsal, or aboral 

 region, one at about a third of the animal's length 

 from each end. In diastole they are nearly 

 spherical, but at the moment of systole, or con- 

 traction, they become stellate, and canals can 

 then be- seen radiating from them. They also 

 open to the exterior at the same moment. 



e. The oral groove runs obliquely backwards along the 



ventral surface from near the anterior end to the 

 mouth, which is placed a little behind the middle 

 of the length of the animal. Its cilia are directed 

 towards the mouth, and drive water and food- 

 particles into it. 



