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THE AMERICAN NATURALIST [Vol.XLVI 



formed a regular part of the progeny of the massive 

 crustacean-feeders. All but invisible, they swam rest- 

 lessly about seeking for available food, which was not 

 present, until many of them fell victims to the greedy 

 members of the parental stock. 



This combination of overfed parent and foodless 

 progeny offered the suggestion of the cause I was seek- 

 ing, which was then readily confirmed by experiment. 

 Maximum nutritive conditions before birth and the entire 

 absence of available food for at least 24 hours after birth 

 produces the slender transparent type with the hyper- 

 trophied humps. Under these conditions the body wall, 

 and its projections, which are highly developed even at 

 birth, continue to develop for a considerable time after- 

 wards, undoubtedly withdrawing nutrition from the in- 

 ternal organs — stomach, digestive glands, ovary, etc. — 

 these thereby undergoing a partial atrophy. 



A certain interest attaches to this explanation, because 

 it not only furnishes the rationale of an extreme type of 

 fluctuating variation in this rotifer, but because the facts 

 closely parallel the incidents in the development of the 

 male of the same species. The males at birth lack, of 

 course, the chief internal organs of the female, and 

 can not draw upon them as sources of nutrition, but they 

 do draw upon the rudimentary digestive tract until, 

 before death, it has frequently quite disappeared. More- 

 over, the males undergo a progressive development of 

 the body wall and to some extent of the humps during 

 the two to four days of their active life. The male thus 

 becomes more differentiated in the active portion of its 

 organization, absorbing meanwhile what little inactive 

 tissue there is to absorb. The same thing happens to the 

 young foodless female, save that there is more tissue to 

 absorb, and the process is not carried so far. 



Sufficient investigation would doubtless unravel each 

 of the other minor fluctuations which the three forms of 

 the species undergo, and most of them will all but cer- 

 tainly resolve themselves into factors of nutrition. Few 



