657 



save that the bodies of the Paramecia are very white, transmitting 

 much light. Watching quiet specimens, the discharge of the contrac- 

 tile vacuoles to the outside is observable with the greatest ease. The 

 clear fluid from within the vacuoles passes into the black and opaque 

 surrounding water, where it is absolutely conspicuous, as a white and 

 shining spot. Owing to the ciliary movement of the animal, the fluid 

 from the vacuole is of course quickly carried away and lost, but is 

 visible for one or two seconds after its discharge. The figure shows 

 the typical appearances as observed in Paramecium. 



There are two chief difficulties in making the observations. First, 

 the Paramecia are of course usually in rapid movement, revolving on 

 the long axis, as well as swimming forAvard. Second, the vacuoles are 





Discharge of the contractile vacuoles in two slightly compressed specimens of Para- 

 mecium, as seen when mounted in India ink. 



much more likely to be discharged on the upper or lower surface of 

 the animals than at the side, and it is only in the latter case, when a 

 profile view is obtained, that the discharge is clearly observed. When 

 the fluid is discharged in such a way as to lie above or below the body 

 of the animal, it is practically invisible, since there is no contrast 

 between it and its background. The simplest way to meet these diffi- 

 culties is to draw some of the water from the preparation by means of 

 filter paper, till the cover glass re-its upon the Paramecia^ compressing 

 them lightly, and holding them still. Usually some of the animals 

 come to rest in such a position that the contractile vacuoles appear at 

 one edge, and are discharged freely into the water, as in the figures. 

 The fluid at the moment of discharge and for a second after is then 

 conspicuous. In order to determine with certainty whether the dis- 



