396 ANN BISHOP 



has none of the coarsely vacuolated structure characteristic of 

 the endoplasm. 



The contractile vacuole lies at the posterior end of the 

 animal. There is a long feeding canal stretching from the 

 anterior end of the animal to the contractile vacuole into 

 which it opens. When distended with fluid the contractile 

 vacuole fills almost completely the posterior end of the animal, 

 and only a very narrow band of endoplasm lies along one side 

 of it, down which the food passes to the median cytopyge (see 

 p. 408). The relative size of the vacuole to that of the whole 

 body, and also its shape, varies with the variety of Spiro- 

 s t o m u m a m b i g u u m , and this point will J^e dealt with 

 later. 



Transverse sections of 8 p i r o s t o m u m a m 1) i g u u m show 

 that the outline of the contractile vacuole and its canal is 

 perfectly definite. The vacuole and its canal lie immediately 

 below the ectoplasm (see Text-figs. 1 and 2). When full the 

 canal protrudes far into the endoplasm, by which it is almost 

 completely surrounded. 



When contraction of the vacuole is about to take place 

 the liquid passes down the feeding canal, which normally 

 closes behind it, and into the vacuole proper, which becomes 

 very much distended. Normally the voiding of the contents 

 to the exterior immediately follows this, but in some cases, 

 particularly in partially narcotized animals, and also often 

 in animals kept for a long time in hollow slides with a small 

 quantity of liquid, evacuation does not immediately take place. 

 The animal continues to swim about with a large, closed vacuole 

 at the posterior end. When the vacuole is about to be emptied an 

 opening is formed at the posterior end at the base of a slight 

 depression ; the body-walls surrounding the vacuole contract 

 from before backwards, the liquid is forced out of the opening 

 and the vacuole disappears. This complete contraction of the 

 contractile vacuole gives the posterior end of the animal a 

 compressed appearance. The new vacuole and feeding canal 

 are formed in exactly the same position as was occupied by the 

 preceding one. 



