504 JOHNSON. [Vol. VIII. 



evidently originates from one of the vacuoles which com- 

 monly lie in a row posterior to the contractile vesicle and often 

 extend into the peduncle. At a point always a little above 

 the level of the posterior extremity of the new zone, one of 

 these vesicles takes on the contractile function and at once 

 acquires the excretory pores (Fig. 26, ex. p.) originally described 

 by Moxon ('69). It contracts at regular intervals, but not 

 synchronously with the old vacuole. 



The formation of the new vacuole in 5. roeselii is brought 

 about by a local dilatation of the longitudinal canal. It seems 

 to have been first observed by Balbiani ('81, p. 322), but was 

 studied independently by me while still in ignorance of 

 Balbiani's mention of it. The dilatation of the canal occurs 

 early in fission, preceding the formation of the mouth, and 

 occupies the same relative position as the new contractile 

 vacuole in S. cceruletis (Fig. 29, c.v.'^). The new vacuole retains 

 its connection with the portion of the longitudinal canal 

 posterior to it, and for a time with the anterior portion also. 



The further history of the excretory system of 5. roeselii at 

 time of fission offers a sufficient explanation of the much- 

 discussed "ring-canal" discovered by Lachmann and figured in 

 Claparede and Lachmann' s great work on the Protozoa ('59). 

 The "ring-canal," encircling the anterior end of the animal, 

 just under the adoral zone, has been looked for in vain by 

 nearly all modern micrographers. But Maupas ('83, p. 641) 

 claims to have seen it in 6". ccenileiis, and Biitschli states ('89, 

 p. 1443) that unpublished drawings of Engelmann (1861) show 

 the ring-canal in 6". ccendeiis and vS. roeselii. 



The study of vS. roeselii in fission gave me the true history 

 of the ring-canal, as the diagrams in Fig. II will show. It is 

 seen that the ring-canal {re) is that portion of the longitu- 

 dinal canal lying immediately anterior to the new contractile 

 vacuole {cv). During the earlier stages of fission {a) it runs 

 straight forward to the old vacuole ; but as the new zone 

 curves round and takes a more nearly horizontal position, the 

 adjacent portion of the longitudinal canal curves with it, and in 

 doing so has to lengthen (b). The newly-formed ring-canal is 

 finally severed from the more anterior portion of the longitu- 



