178 ANIMALS AND PLANTS VI 



But other active organisms, very much larger 

 than the Bacteria, attaining in fact the com- 

 paratively gigantic dimensions of -^-oVo of an 

 inch or more, incessantly crossed the field of view. 

 Each of these had a body shaped like a pear, the 

 small end being slightly incurved and produced 

 into a long curved filament, or cilium, of extreme 

 tenuity. Behind this, from the concave side of 

 the incurvation, proceeded another long cilium, 

 so delicate as to be discernible only by the use of 

 the highest powers and careful management of 

 the light. In the centre of the pear-shaped 

 body a clear round space could occasionally be 

 discerned, but not always; and careful watching 

 showed that this clear vacuity appeared gradually, 

 and then shut up and disappeared suddenly, at 

 regular intervals. Such a structure is of common 

 occurrence among the lowest plants and animals, 

 and is known as a contractile vacuole. 



The little creature thus described sometimes 

 propelled itself with great activity, with a curious 

 rolling motion, by the lashing of the front cilium, 

 while the second cilium trailed behind ; some- 

 times it anchored itself by the hinder cilium and 

 was spun round by the working of the other, its 

 motions resembling those of an anchor buoy in a 

 heavy sea. Sometimes, when two were in full 

 career towards one another, each would appear 

 dexterously to get out of the other's way ; some- 

 times a crowd would assemble and jostle one 



