I PHYLUM PROTOZOA 45 



animalcule," a Protozoan of comparatively large size, about 

 | mm. in length, which moves about very actively like 

 Euglena, but with a more regular and more rapid move- 

 ment and by means of organs of locomotion differing in 

 character from the flagellum of the latter. The body of 

 Paramcecium (Fig. 18, A, B} is covered with what appear 

 under the microscope like small delicate hairs arranged in 

 longitudinal rows. These are the cilia ; they are in inces- 

 sant to-and fro vibration, and it is by their means that the 

 Paramcecium moves about and obtains its food. In shape 

 the body is somewhat cylindrical, rounded at the anterior 

 and bluntly pointed at the posterior end. On one side, 

 the ventral, is a large oblique depression, the buccal groove 

 (hue. gr.\ leading into a short gullet (gul.\ which, as in 

 Euglena, ends in the soft internal protoplasm. The proto- 

 plasm is differentiated into a firmer superficial layer, the 

 cortex (corf.), and a semifluid central mass, the medulla 

 (wed.), and is covered superficially by a thin cuticle. The 

 cilia are prolongations of the cortex, and perforate the 

 cuticle. 



In the cortex are found two nuclei. One of these, the 

 meganuclcus (mt.\ is a comparatively large ovoid body ; the 

 other, the micronucleiis (pa. nu.\ is a small rounded body 

 closely applied to the meganucleus. Two contractile 

 vacuoles (c. vac.} are present. Each is connected with a 

 series of radiating spindle-shaped cavities in the protoplasm 

 which serve as feeders to it ; after the contraction of the 

 vacuole these cavities are seen gradually to fill, apparently 

 receiving water from the surrounding protoplasm ; they then 

 contract, discharging the water into the vacuole, the latter 

 rapidly enlarging while they disappear from view ; finally 

 the vacuole contracts and discharges its contents externally. 



The cortex contains minute radially-arranged sacs called 



