SPIROSTOMUM AMBIGUUM 395 



to use a hollowed slide, which, when it was not under observa- 

 tion, was kept in a moist chamber. Although hanging drops 

 were used at first, they were abandoned later, when it was 

 found that when the animal moved to the edge of the drop, 

 which it normally did, it rapidly disintegrated there. 



To facilitate observations on living Spirostoma, Caragheen 

 extract was used. This slows down their movements con- 

 siderably ; but, since their shape becomes somewhat distorted 

 with the density of the medium and disintegration often 

 follows, it is not advisable to use it when keeping the animals 

 under observation for a long period. It was very useful, 

 however, for the study of ciliary structures. 



Feeding M e t h o d s . — For following the course of ingested 

 material, finely powdered carmine or Indian ink in culture 

 solution were both used. A dilute solution of milk in culture 

 solution was also used, as was also finely powdered yolk of egg. 



3. Observations on the Morphology of Spirostomum 

 ambiguum. 



As a description of the general morphology and movements 

 of Bpirostomum ambiguum Dr. Stein's (28) excellent 

 account has not been improved upon. It will be sufficient 

 here, before passing on to a detailed account of the 

 various structures, to mention that Spirostomum ambi- 

 guum is a large, elongated ciliate belonging to the order 

 Heterotricha. The peristomial groove, which terminates in 

 the mouth, is lateral in position, but the distance of the mouth 

 from the anterior end of the body may vary considerably in 

 different individuals. The peristomial membranellae extend 

 from the extreme anterior end to the mouth, around which they 

 curve in a spiral manner. The meganucleus is long and monili- 

 form. The numerous small micronuclei are situated close to 

 the meganucleus. 



Contractile Vacuole . — S p i r o s t o m u m ambiguum 

 is bounded externally by a relatively thin layer of ectoplasm, 

 on the outer side of which is the thin cuticle. The ectoplasm 



