X CONTRACTILE VACUOLES in 



minute size make them somewhat difficult to see while the 

 Paramoecium is alive and active, but after death they are 

 very obvious, and look quite like a thick covering of fine 

 silky hairs. 



Near the middle, and close to the cortex, is a large oval 

 nucleus (B, nu), which is peculiar in taking on a uniform tint 

 when stained, showing none of the distinction into chroma- 

 tin and nuclear sap which is so marked a feature in many of 

 the nuclei we have studied (see especially Fig. i, p. 2, and 

 Fig. -9, p. 62). It has also a further peculiarity: against one 

 side of it in P. caudatuvi is a small oval structure {pa. nii) 

 which is also deeply stained by magenta or carmine. This 

 is the micronuckus : it is to be considered as a second, 

 smaller nucleus, the larger body being distinguished as the 

 meganucleus. 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 are in relation with the cortex. 



The action of the contractile vacuoles is very beautifully 

 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 

 supended 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, spindle- 

 shaped spaces filled with fluid appear round it, like the rays 

 of a star (upper vacuole in a & b) : the vacuole itself contracts 

 or performs its systole, completely disappearing from view, 

 and immediately afterwards the radiating canals flow together 

 and re-fill it, becoming themselves emptied and therefore 

 invisible for an instant (lower vacuole in .\ & b) but rapidly 



