Phylum Protozoa 369 



tween the bases of the cilia. When the animal is attacked or disturbed 

 in any way, these are discharged as long threads. 



Toward the anterior end there is a shallow depression, the oral 

 groove. The beating cilia tend to sweep food into this depression which 

 terminates in the cytostome or cell month. A slender tube, the cyto- 

 pharynx, leads into the endoplasm where the food vacuoles are formed. 

 Along the cytopharynx the cilia fuse to form two undulating membranes. 

 Food vacuoles formed at the cytopharynx may be seen circulating 

 through the endoplasm as digestion progresses. The constant flowing 

 or cyclosis of the cytoplasm carries the various cell inclusions about. 

 Posterior to the cytopharynx is the cell anus through which solid waste 

 particles are discharged from time to time. At either end of the body 

 a large contractile vacuole may be seen to pulsate as excess liquids are 

 discharged. Several canals, the radiating canals, lead from certain 

 regions of the endoplasm to the contractile vacuoles and bring fluids 

 to the vacuole for discharging. 



Two nuclei are present : the macronucleus and the micronucleus. 

 The macronucleus, in general, controls the cellular metabolism, that is, 

 the vegetative functions of the animal. The micronucleus is concerned 

 with the reproduction and carries the genes or hereditary material. 



The beating of the cilia is such that the animal actually swims with 

 a spiral motion. Like most protozoans, they demonstrate avoiding re- 

 actions to various chemicals. If they come in contact with an acid, they 

 are able to reverse the action of the cilia and back away. A weak acid 

 placed on a slide will cause the animal to discharge its trichocysts as 

 well as to move away. On the whole, the paramecia avoid extremes 

 of temperature and light, apparently finding a moderate amount most 

 favorable. 



Binary Fission. — Paramecium has a complex life cycle showing 

 both sexual and asexual methods of reproduction. The commonly 

 observed type of reproduction is the asexual in which the individual 

 reproduces by transverse binary fission. At the beginning of binary 

 fission, the macronucleus divides by amitosis, simply elongating and 

 pulling apart. The micronucleus divides by means of modified mitosis. 

 As the new cells pull apart, a constriction appears and the animal is 

 gradually cut into two parts. Each new cell will have certain structures 

 which happened to be in its half, and, of course, a full complement of 

 chromosomes. The missing structures are soon regenerated. In cul- 

 tures, this type of reproduction may continue for long periods. 



