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THE AMERICAN NATURALIST [Vol. XL VI 



number of instances, by thoroughly conservative counts, 

 I reached the surprising number of 115. The nephridial 

 development in this giant rotifer is therefore in propor- 

 tion to its large size and massive tissues. The trophi 

 also proved sufficiently different and of a size hitherto 

 quite unknown in the genus. A glance at Figs. 3 and 4, 

 showing the trophi of the two types drawn to the same 

 scale, will show at once their great difference. A detailed 

 discussion of their minor features will show them to be 

 still further apart. 



In general, then, to judge by the structure of the 

 females, the two types seemed very distinct, and several 

 weeks of painstaking study, including the examination of 

 several thousand individuals, left me with very little 

 doubt that I was dealing with what we ordinarily class as 

 distinct species. 



It was while examining some of my first mounted slides 

 of the large Asplanchna that I came suddenly on the evi- 

 dence of apparent mutation. As is well known, the 

 young of Asplanchna are highly developed before birth, 

 and in the material which I was studying this chanced 

 to be true to an unusual degree. My mounted slides, too, 

 were perfectly transparent, giving views not only of alt 

 the organs of the adult, but of every organ and almost 

 every histological detail in the structure of the unborn 

 young. Examining these young, I noticed at first noth- 

 ing peculiar ; the young within the humped rotifers bore 

 humps even more marked than those of the adult, it being 

 one of the recognized characters of A. amphora that these 

 body extensions reach a maximum development in the 

 young at about the time of birth. The first young noted 

 in the campanulate type were also, in all respects, essen- 

 tia I ]>■ like the parents. 



I was, therefore, astonished to suddenly hit upon a 

 large campanulate rotifer containing an unborn rotifer, 

 not of its own, but of the other, the humped type, Fig. 2. 

 The word mutation was the first thing that framed itself 

 in the midst of my inarticulate consciousness. But ot 



