AMCEBA AND SOME ALLIES 293 



As in Amoeba and Euglena, the entire body of Para- 

 mecium is a single cell. The cell when moving freely has a 

 definite shape, although it changes constantly in outline, 

 owing to the fact that the irregularities in the body are 

 whirled into view as the animal swims along in a slender 

 spiral path. 



Paramecium is about 0.2 mm. (xW5 m m length. The 

 anterior end is rounded and the posterior end pointed. 

 Right and left sides are, as indicated in the figures, deter- 

 mined by the position of the mouth (Fig. 152, 2), which is 

 on a surface called the oral surface. The entire surface of 

 the ectoplasm is covered with great numbers of short, hair- 

 like structures, called cilia (Fig. 152, 1). The cilia are the 

 organs of locomotion. They wave backward toward the 

 posterior end, propelling the animal forward. A few cilia 

 somewhat longer than the others lie in the groove that leads 

 diagonally across the ventral surface to the mouth. Their 

 function is to carry the food down the short gullet into the 

 endoplasm. 



The endoplasm, which, as already explained, is that por- 

 tion of the cytoplasm lying between the nucleus and the out- 

 side layer of ectoplasm, is soft and semifluid. Food is passed 

 into it by the gullet (Fig. 152, 3), and immediately begins to 

 float away from that point, surrounded by a little drop of 

 water. These masses are called food vacuoles (Fig. 152, 4, 5). 

 While the current in the cell protoplasm is carrying the food 

 vacuoles around, digestive fluid formed by the protoplasm 

 is breaking up the food, liquefying it, and changing it chem- 

 ically for the process of assimilation or building up into pro- 

 toplasm. The indigestible particles are discharged from the 

 cell by a small opening, the anus (Fig. 152, 6). The waste 

 derived from the food and protoplasm that is used up in the 

 work of the animal is probably discharged in the form of 

 liquid from two special organs, one near either end at the 



