410 ANN BISHOP 



the major variety the mouth may be situated almost at the 

 extreme posterior end, about level with the middle of the con- 

 tractile vacuole. This variation in the position of the mouth 

 depends, as will be seen later, upon the degree of growth 

 to which the animal has attained since the last fission. In 

 the minor variety the peristomial membranellae extend from 

 the extreme anterior end backwards to the mouth, which 

 usually is in the anterior third of the body length. 



The shape of the contractile vacuole also differs in each 

 form. In Spirostomum ambiguum major it is a 

 pear-shaped vessel, almost as broad as the animal, but never 

 occupying more than an eighth of the animal's length. In 

 the minor variety it is much larger, generally filling, when fully 

 expanded, the posterior quarter or even third of the animal. 

 In some cases it has been observed to fill half the total length. 

 The shape of the contractile vacuole in the minor variety tends 

 towards an oblong. 



The time between contractions of the vacuoles in the two 

 varieties varies. Thus, in the major variety the average 

 time obtained from repeated observations on a large number 

 of animals from four different cultures was every eight and 

 a half minutes. The maximum period between contractions 

 which was ever observed was ten minutes and the minimum 

 seven minutes. In the minor variety the average was sixteen 

 minutes. On two occasions thirty and thirty-four minutes 

 respectively elapsed between contraction of the vacuoles of 

 individuals of this variety. Since these two figures were 

 so different from the rest they were not included in the average. 



The meganucleus in both varieties is similar in form and 

 structure, but I have never seen large, multi-vacuolated 

 macrosomes in the meganucleus of the small variety. Since 

 the_ contractile vacuole is very large, the length of the mega- 

 nucleus in proportion to the body length in the small variety is 

 less than in the large variety. 



The micronuclei in the small variety are similar to those 

 of the majority variety except that they are smaller and often 

 rather disc-shaped. 



