SPIROSTOMUM AMBIGUUM 409 



move much more rapidly, and are therefore expelled in a much 

 shorter time than nutritious ones. 



A suspension of hard-boiled yolk of egg in water was fed 

 to some Spirostoma. The particles ingested were observed 

 to follow the course taken by carmine granules and Indian ink. 

 A similar course was followed by globules of raw milk. From 

 this it seems apparent that raw milk and yolk-granules are 

 not nutritious to Spirostomum ambiguum, the animal 

 evidently being unable to digest them. 



6. Varieties of Spirostomum ambiguum. 



During the observations made upon a number of different 

 cultures of Spirostomum ambiguum it soon became 

 evident that two distinct varieties were present. That these 

 were not stages in a developmental cycle was proved by making 

 pure cultures of each. 



Stein (28) describes a number of varieties of which his 

 PI. ii, fig. 10, and PI. iii, fig. 3, show two chief types. These 

 two main varieties, corresponding to the ones present in the 

 cultures, are also recognized l3y Ptoux (26), and are termed 

 by him Spirostomum a m b i g u u m m a j o r and Spiro- 

 stomum ambiguum minor. These two varieties differ 

 from one another in a number of important details, the most 

 striking of which is size. 



The major variety is usually much broader in proportion 

 to its length than the latter. The average length of the ordinary 

 members of the major variety, Avhen fixed with warm Schau- 

 dinn's solution, is 800-900 [x, whilst that of ordinary members 

 of the minor variety, when treated in the same way, is 400- 

 500 jx. The posterior end of the minor variety is truncated, 

 whilst that of the major variety is rounded. The protoplasm 

 of the major variety is yellowish in colour, whilst that of the 

 minor variety is greyish white and the endoplasm is less 

 coarsely vacuolated and more granular in the latter than in 

 the former. In the major variety the peristomial membranellae 

 extend from the extreme anterior end to some point posterior 

 to the middle of the animal's bodv. In some individuals of 



