in STRUCTURE 265 



The mouth (mtti) leads into a short funnel-like 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 mouth (p. 109), and are thereby 

 distinguished from the flagella of Sphaerella, Euglena, &c., 

 which exhibit more or less intermittent lashing move- 

 ments. 



Near the middle of the body, on the inner boundary of 

 the cortex, is a large oval nucleus (B, c, nu\ and 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 micronucleus : it is to be considered 

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

 tinguished as the meganucleus. In the closely allied 

 P. aurelia, there are two micronuclei. 



There are two contractile vacuoles (c. vac) in relation with 

 the cortex, 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. 



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 

 suspended its active, swimming movements. It is then seen 

 that during the diastole, or phase of expansion of each 

 vacuole, a number- about six to ten of delicate, radiating, 



