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THE AMERICAN NATURALIST [Vol. XLV1 



production. Thus in the case of one pond, an algal- 

 feeding Brachionus developed until it exceeded in num- 

 bers anything that I had ever before observed under sim- 

 ilar conditions. The surface water was so filled with 

 them that one observer pronounced it as thick as good 

 tomato soup. In this medium the humped Asplanchna 

 gorged itself during days of rapid multiplication. Every 

 stomach was packed with the smaller rotifers; yet no 

 change in type resulted. Finally the Asplanchnas liter- 

 ally ate up the Brachionus, but themselves disappeared 

 a short time after this was accomplished without having 

 produced any of the giants. Yet condensed material 

 taken from this same source and placed in large culture 

 dishes did produce the cannibals some time after the 

 Brachionus had been devoured. It follows from this in- 

 stance, as well as from a number of others equally con- 

 clusive, that mere feeding on the flesh of rotifers, so to 

 speak, is not sufficient to cause the change. 



The same proved true of over-feeding on Paramecium 

 and several other protozoa that were greedily eaten. 



I was the more surprised, therefore, to discover one 

 other food organism, and one only, so far as my observa- 

 tions extend, which is fully competent to bring about the 

 same result. It is the Daphnid-like crustacean, Moina 

 paradoxa. It seems almost incredible that a rotifer 

 should feed upon this robust entimostracan, even the 

 young of which are born of a size which would seem be- 

 yond the utmost stretch of a rotiferean stomach. More- 

 over, this Asplanchna does not by any means invariably 

 attack Moina. It may disappear, seemingly from starva- 

 tion, in a pond where Moina is present and rapidly mul- 

 tiplying. This has also happened repeatedly in my cul- 

 tures, and I have placed numbers of the young Moina s in 

 watch-glasses together with the Asplanchna without hav- 

 ing them eaten. Yet in other very similar cases the 

 young Moinas are ingested by the Asplanchnas— -by the 

 humped form, and rarely by the larger saccates. The 

 very youngest Moinas may even at times be ingested by 



