60 ORGANISMS OF ONE CELL 



ing into a short pharynx 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 the cells are much more differentiated into 

 cellular organs or organelles. 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 

 that causing pseudopodia formation in Amoeba. Nuclei and 

 contractile vacuoles are quite different and more complex than 

 in Amoeba or Chilomonas. 



Nuclei. Paramecium and the Infusoria generally, are differ- 

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

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

 conspicuous; the other, the micronucleus is very small &nd usu- 

 ally 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 the 

 reproduction and the maintenance of the race. 



Contractile Vacuoles. The contractile vacuole of Amoeba 

 proteus is a single spherical vesicle which moves in the endo- 

 plasm with the other endoplasmic organs. Paramecium is more 

 highly differentiated in this respect by having two vacuoles 

 which are fixed in the cell opening to the outside by permanent 

 pores in the membrane. A conspicuous feature in regard to 

 them is that special canals feed them by bringing waste matters 

 from all parts. When these canals are full a characteristic 

 radiate structure about the vacuole can be made out with ease. 

 One of the vacuoles is in the anterior third of the cell the other 

 in the posterior third. Systole or rupture of the vacuole and 



