HI STRUCTURE 265 



The mouth (nith) leads into a short funnel-Hke tube, the 

 gullet (gut), which is lined by cuticle and passes through the 

 cortex to end in the soft medulla, thus making a free com- 

 munication between the latter and the external water. ,., 



The cilia with which the body is covered are of approxi- 

 mately equal size, quite short in relation to the entire animal, 

 and arranged in longitudinal rows over the whole outer sur- 

 face. They consist of prolongations of the cortex, and each 

 passes through a minute perforation in the cuticle. They 

 are in constant rhythmical movement, like the cilia on the 

 epithelial cells of the frog's ntouth (p, 109), and are thereby 

 distinguished from the flagella of Hsematococcus, Euglena, 

 &c., which exhibit more or less intermittent lashing move- 

 ments. 



Near the middle of the body, in the cortex, is a large oyal 

 nucleus i^ynu) which is peculiar in taking ori a uniform tint 

 when stained, showing none of the distinction into chroma- 

 tin and achromatin which is so marked a feature in many of 

 the nuclei we have studied. It has also a further peculiarity : 

 against one side of it in P. caudatum is a small oval structure 

 (pa. nu) which is also deeply stained by, e.g., magenta or 

 carmine. This is the micronudeus : it is to be considered 

 as a second, smaller nucleus, the larger body being dis- 

 tinguished as the meganudeus. In the closely allied 

 P. aurelia, there are two micronuclei. 



There are two contractile vacuoles {c. vac), one situated at 

 about a third of the entire length from the anterior end of 

 the body, the other at about the same distance from the 

 posterior end : they occur in the cortex. 



The action of the contractile vacuoles is very beautifully 

 seen in a Paramcecium at rest : it is particularly striking in a 

 specimen subjected to slight' pressure under a cover glass, 

 but is perfectly visible in one which has merely temporarily 



