THE PROTOZOA 



A. General appearance. 



The animal is an elongated, somewhat flattened body 

 about T J~o mcn i n length, rounded at its narrower anterior 

 end, and bluntly pointed at the broader posterior end. 



It swims actively by means of cilia distributed over its 

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

 is an oblique groove leading to the mouth. 



AV 



PV 



TR 



MY 



EN EC FV 



YIQ. 2. Paramecium aurelia seen from above and slightly from 

 the right side ; x 300. (c. H. H.) 



AV, anterior pulsating vacuole. EC, ectosarc : the lines running across 

 it indicate the trichocysts. EN, endosarc. EP, paranucleus. FV, food- 

 vacuole. M, mouth. MY, 'myophan' striation. N, nucleus. OG, oral 

 groove. PV, posterior pulsating vacuole in systole. TK, discharged tricho- 

 cyst threads. X, cilia. 



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. 



a. The cuticle is the delicate superficial and stiffer 



layer, serving as a protective covering for the 

 underlying protoplasm by which it is secreted. 



b. The cilia are very numerous delicate vibratile fila- 



ments arising from the layer of ectosarc imme- 

 diately beneath the cuticle, through which they 

 project. They are of uniform size over the entire 

 surface, and extend into the oral groove. Their 



