Introduction. 



X^rom the moment when the microscope was invented, the Rotifera have in- 

 terested the naturahsts in a verj' higli degree. They were studied by the great na- 

 turaHsts of the eighteenth century together with the innumerable unicellular organisms 

 which, in a single drop, were brought under the microscope in many hundreds of 

 specimens. As well known, our own countryman O. F. Müller (1786) has contributed 

 in a very great degree to the study of these charming little creatures. Even if some 

 of these old naturalists had a conception that Infusoria and Rotifera really only 

 were identic with regard to size, but differed very much from each other in all 

 essential points, they were actually, until 1838 when Ehrenberg's famous work arrived, 

 treated in the same great division of lower organisms, which the different authors 

 gave very different names. 



It was Ehrenberg who was the first to definitively separate the Rotifera from 

 the Protozoa; as of course at the then existing stage of science he could have no 

 idea of the fact that the first group comprised multicellular, the other unicellular 

 organisms, he had also no idea of how great the gulf between the two divisions 

 really was. The word Protozoa was first formed by v. Sieboed in 1845, the word 

 Metazoa by Haeckel, and he is really the first to contrast the Protozoa with "die 

 Gewebetiere" (Metazoa). Until 1838 all described and figured Rotifers have most 

 probablj' only been females. Almost all the hundreds of Rotifers which Ehrenberg 

 figured and described in such an admirable manner were females. But Ehrenberg 

 did not regard them as such, but as hermaphrodites; according to him the excretory 

 organ with the lateral canals and contractile vesicle were the male organs. It was 

 later on shown that two of his Rotifers, Enteroplea hgdatina and Notommata granu- 

 lans, were really Rotifer males, but he was so convinced of the correctness of his view 

 of the hermaphroditic character of the group, that the two above named males, 

 the first-named the male of Hydatina senta, the last-named the male of Brachionus 

 pala, were described as Rotifers, quite like all the other Rotifers and regarded as 

 hermaphroditic like these. Already the naturalists of the time in which Ehrenberg 

 lived, understood that the organs which Ehrenberg described as male organs, could 

 not be interpreted as such; on the other hand true male organs could not be found. 

 In his "Vergleichende Anatomie" Sieboed (1848, p. 184) is therefore forced to confess: 

 „Trotz der sorgfältigsten Bemühungen hat sich bis jetzt kein befriedigendes Resultat 



26* 



