J: 



98 THE SIMPLEST ORGANISMS 



cells in the leaf. The bacteria themselves release this food from 

 the hay by causing it to decay. 



Where to find Paramecia. — If we examine the surface of a 

 hsij infusion, we find a scum formed of bacteria, and a mass of 

 whitish tiny dots collected along the edge of the jar close to the sur- 

 face of the water. More attentive observation shows us that these 

 objects move, and that they are never found far from the surface. 

 They are one-celled animals of several species, but among them 

 we are almost sure to find a slipper-shaped cell, the parame'cium. 

 The Structure of a Paramecium. — The cell body is almost 

 transparent and consists of semifluid protoplasm which has a 



granular, grayish appearance under the 



^j^Peiiide microscope. This protoplasm appears to 



^Contractile vacuole ^^ bounded by a very delicate peVlicle or 



covering through which project numer- 



Micronuchus ous delicate threads of protoplasm called 



Mouth cilia. The locomotion of the Paramecium 



\ ^^ - f^ood vacuole is caused by the movement of these cilia, 



. . , which lash the water like a multitude of 



. "- ,'"' ContracHle vacuole ^^^J ^^^^- The cilia also send particles of 



^ food along a groove into a funnel-like 



%/,i'^' opening, the gullet, on one side of the cell. 



A Paramecium. Once inside the cell body, the particles of 



food materials are gathered into little balls within the almost 



transparent protoplasm. These masses of food are inclosed in a 



little bubble-like area called a vacuole, containing fluid. Two larger 



vacuoles may be found; these are the contractile vacuoles; their 



purpose seems to be to pass off Hquid waste material from the cell 



body. This is done by pulsation of the vacuole, which ultimately 



bursts, passing fluid waste to the outside. Solid wastes are passed 



out of the cell through an a^nal opening, in somewhat the same 



manner. No breathing organs are seen, because diffusion of 



oxygen and carbon dioxide may take place anywhere through the 



cell membrane. The nucleus of the cell is not easily visible in 



living specimens. In a cell that has been stained it has been found 



to be a double structure, consisting of a large and a small portion, 



called, respectively, the macronucleus and the micronucleus} 



^ Some species of paramecia have two micronuclei. 



