288 



GENERAL ZOOLOGY 



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, 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. 143, 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 dorsal surface. 

 These are contractile vacuoles (Fig. 

 143, 7). It is supposed that the liquid 

 waste when formed, flows through the 

 protoplasm along somewhat definite 

 channels, to the point where it collects 

 in a gradually increasing vacuole. 

 Soon the maximum size is attained, 

 and the vacuole bursts at the surface, 

 the waste pouring into the water out- 

 side. The two contractile vacuoles 

 alternate in contracting. 



Paramcecium has two nuclei ; the 

 large one is called the macrvnucleus 

 (Fig. 143, 8), and the small one the 

 micro nucleus (Fig. 143, 9). The macro- 

 nucleus is thought to be the seat of 

 the general activities of the cell, while 

 the micronucleus is the seat of the 

 important process of reproduction. 



The theory that reproduction by divi- 

 sion is a phase of growth was stated in 



the description of Amoeba. Paramoecium also divides into 

 two equivalent cells (Fig. 144), with half the macronucleus 

 and half the micronucleus in each, probably because the 



FIG. 144. Paramoecium 

 dividing. Much en- 

 larged 



1, mouth and gullet; 

 2, macronucleus divid- 

 ing; 3, micronucleus 

 dividing 



(From Sedgwick and Wil- 

 son's General Biology) 



