CONTRACTILE VACUOLE 109 



ments (see p. 25, note, and p. 59). Their rapid motion and 

 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 of the body, in 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 achromatin 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 is a small oval structure (pa, mi) which is also deeply 

 stained by magenta or carmine. This is the paramicleus ; it 

 is to be considered as a kind of second, smaller nucleus. 



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 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 thus seen 

 that during the diastole, a 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) but rapidly appearing 

 once more. There seems to be no doubt that the water 

 taken in with the food is collected into these canals, emptied 



