PARAMECIUM CAUDATUM 61 



synchronous beating of the water by myriads of minute cilia 

 uniformly distributed over the surface in diagonal lines, while 

 the rotation on the long axis is due partly to this arrangement 

 and partly to the action of cilia covering an asymmetrical 

 groove called the peristome, extending from the anterior end 

 backward to about the middle of the body. This peristomial 

 area does not run in a straight line from the anterior end to 

 the center but curves from the left dorsal extremity to the 

 right, ending to the left of the middle of the ventral surface 

 at the mouth. The area deepens from in front backward, until 

 at the mouth a conspicuous pocket is formed (Fig. 26). The 

 mouth is a circular opening leading into a short gullet which 

 bears an undulating membrane on one side. 



Structure. The finer structure of Paramecium differs con- 

 siderably from that of Amoeba and Chilomonas owing to 

 the fact that Paramecium cells are much more highly differen- 

 tiated into cellular organs. We can recognize, however, a 

 distinct endoplasm and ectoplasm and note that the chief 

 differentiations are in the latter. The endoplasm is made up 

 of alveoli similar to those of Amoeba, and we find the same 

 granular microsomes and larger food particles in various stages 

 of digestion. The protoplasm also undergoes streaming move- 

 ments or cyclosis, the movement being entirely within the cell, 

 however, and irregular so that it appears to be different from 

 the movement involving pseudopodia formation in Amoeba. 

 Nuclei and contractile vacuoles are quite different and more 

 complex than in Amoeba or Chilomonas. 



Nuclei. Paramecium and the Infusoria, in general, are differ- 

 ent from all other cells in having two kinds of nuclei, macronu- 

 clei and micronuclei. One of these, the macronucleus, is large 

 and conspicuous; the other, the micronucleus, is very small and 

 usually partly embedded in the substance of the larger nucleus. 

 While not fully proved it is probable that these two kinds of 

 nuclei have different functions to play in the vital activities in 

 the cell, the macronucleus being the chief seat of metabolic 

 activities, while the micronucleus is mainly concerned with 

 reproduction and maintenance of the race. 



