60 C. F. ROUSSELET ON THE ROTIFERA OF DEVILS LAKE 



Devils Lake in the month of July 1910, and all these had the 

 shape shown in the figure, with two long, curved and widely- 

 separated posterior spines. Last year I obtained from Prof. Young 

 a collection made in the month of May 1912, much earlier in the 

 season, when the weather in North Dakota is still cold and the 

 water chilly. Together with the fully developed forms in this 

 collection I found a much smaller form, with short posterior 

 spines, curved inwards and other unusual features as represented 

 in PI. 6, fig. 26-/. The six frontal spines and the mental edge 

 are identical with those of the larger specimens, but the shape of 

 the body and the form and size of the posterior spines are very 

 different, and, strangest of all, the foot-opening is situated on the 

 postero-dorsal side of the lorica, a quite unheard-of position in 

 this genus. My first impression was that these were young 

 animals just hatched from eggs, but this is evidently not so, for 

 some specimens were seen carrying their eggs at the base of the 

 foot on the dorsal side, and they were therefore adults reproducing 

 freely. I can only conclude that this represents a case of 

 dimorphism, possibly a winter form which gradually, in successive 

 generations, transforms itself into the larger form with extended 

 and expanded posterior spines. In saying this I do not mean 

 that the smaller forms (PI. 6, fig. 26-c) can themselves grow into 

 the form of fig. 2a, but that their offspring will in a few genera- 

 tions more and more resemble the larger form. Intermediate 

 forms between the two types figured were not seen. In order to 

 follow up this transformation it will be necessary to obtain 

 plankton collections made about twice a month throughout the 

 year, which at present are not available. It certainly is not 

 easy to see how the dorsally situated foot-opening can change 

 into the median posterior position of the larger form, but it is 

 known that in the case of some Asplanchna (A. amphora, 

 A. Sieboldii) the transition from humped into saccate forms and 

 other changes take place suddenly, from one generation to the 

 next, produced apparently through a change of diet and 

 temperature, as shown by the recent researches of Dr. Arno 



