LIFE HISTORY OF TWO RARE CILIATES 375 



I 



settling down from time to time to feed; this process which 

 occupies about one hour and a half is preceded by an elonga- 

 tion of the body and nucleus, the difference in width between 

 the anterior and posterior regions becoming more marked as 

 the constriction deepens between the two ends (figs. 54 and 60). 

 The difference in size between the anterior and posterior body 

 is especially well seen when the animal comes to rest mouth 

 downward, the posterior cell resting on top of the anterior one, 

 the peripheries, in optical section, appearing as concentric circles. 

 About half-an-hour after the animal begins to divide, a vacuole 

 is formed anterior to the constriction and a little to one side 

 of the median plane of the body. It is difficult to determine 

 the exact location of the new mouth in the dividing cell, owing 

 to the fact that while the organism is in motion it is impossible 

 to keep it in focus long enough to find the mouth, while, when 

 it comes to rest, this part of the bodj' is directed downward 

 and cannot be seen. When the two cells separate, however, 

 the mouth is readily found at the extreme anterior tip of the 

 body (figs. 55, 56, 57, 58, 59, 61). 



5. Summary 



(a) Actinobolus radians is an almost spherical organism vary- 

 ing in length from 37.5/x, to 77m, in width from 22. 5^ to 66.5^; the 

 average length and width of 25 individuals measured was 58. 5^ and 

 46.8m respectively. It feeds exclusively on Halteria grandinella. 



(b) Two clearly differentiated regions are distinguishable in 

 the living cell, a dense central mass, the endoplasm, surrounded 

 by a semitransparent envelope, the cortex. Large vaculoles 

 form a characteristic peripheral border. 



(c) The cuticle is marked by twenty four or five delicate 

 lines extending spirally from the borders of the mouth to the 

 posterior end of the cell and marking the places of insertion of 

 the retractile tentacles and the cilia. 



(d) The cilia are arranged in clusters of from five to ten at 

 the base of each tentacle and show a flagellum-like motion, 

 rather than the synchronous beating of ordinary cilia. 



