PARAMECIUM. 



entire surface. Near the middle of the ventral or oral surface 

 is an oblique groove, the Vestibule, leading to the mouth. 



B. Structure. 



The two layers, ectosarc and endosarc, are much more 

 sharply denned than in Amoeba. 

 1. The ectosarc is the comparatively firm outer layer, the 

 elasticity of which preserves the general form of the 

 body. Its deeper part is marked by longitudinal 

 or oblique ' myophan ' striations or myonemes, which 

 vary in distinctness with the movements of the 

 animal, and form a contractile system. They are 

 due to differentiation of fibres of the ectosarc. 



a. The pellicle 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. An undulating membrane com- 

 posed of a partially coalesced tine of cilia lies 

 in the vestibule and mouth. This serves 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 pulsating vacuoles are situated in the substance 

 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 



