m STRUCTURE 265 



The mouth (mth) leads into a short funnel-like tube, 

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

 through the cortex to end in the soft medulla, thus 

 making a free communication between the latter and 

 the external water. 



The cilia with which the body is covered are of 

 approximately equal size, quite short in relation to the 

 entire animal, and arranged in longitudinal rows over 

 the whole outer surface. They consist of prolongations 

 of the cortex, and each passes through a minute per- 

 foration in the cuticle. They are in constant rhythmi- 

 cal 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 distinguished as the meganudens. 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 beauti- 

 fully seen in a Paramoecium 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 

 ohase of expansion of each vacuole. a number about 



