276 
BULLETIN OE THE UNITED STATES FISH COMMISSION. 
The classification of the Rattulidcn has fallen into great confusion. This state- 
ment could be made of almost any of the larger groups of Rotatoria, but it is perhaps 
more strikingly true of this family than of any other. Many species have been 
described under several different specific and generic names, while in other cases 
several different species have been described under a single name. The twisted, 
unsymmetrical structure has always been more or less of a puzzle to systematists, 
making it difficult to determine even what were properly to be considered dorsal and 
ventral surfaces, and the great difference in appearance between contracted and 
extended animals has further tended to favor confusion. It has seemed to the writer 
that there is no group of the Rotatoria so much in need of a thorough revision as 
this one. For this reason it has been taken up first. 
In the following paper I attempt to give an account of the structure and move- 
ments of these animals, paying especial attention to the asymmetry and its biological 
significance, and to furnish as far as possible full descriptions and figures of all 
known species. A large majority of the known species I have myself been able to 
study, and in these cases the descriptions and figures are based on my own observa- 
tions. I have attempted to make these so detailed that further mistakes in the 
identification of these species will hardly be possible. In the. case of species which 
I have not been able to examine myself I give the figures and descriptions which 
have been published by other authors. Many of these descriptions are very unsatis- 
factory, as comparison with a large number of species is necessary for bringing out 
the important characteristics, and such comparison has, in the absence of preserved 
material, been almost wholly lacking until very recent times. 
In the preparation of this paper I have been especially indebted for assistance 
of the most essential character to Mr. Charles F. Rousselet, of London, England, and 
to Mr. F. R. Dixon-Nuttall, of Eccleston Park, North Prescot, England. Mr. Rous- 
selet placed at my disposal his valuable mounted collection of the Rattulidcu, includ- 
ing a number of species which I did not have in my collection, and lias assisted me 
throughout the work with valuable notes and suggestions. Mr. Dixon-Nuttall sent 
me li is notes and drawings of a considerable number of species of Dmrellct, which he 
had long been studying, and gave me permission to make use of some of his excel- 
lent figures, a number of which are given on plates iv and xm. The continued 
cooperation of these two careful investigators has added much to the completeness 
and accuracy of this paper, and has made it possible, by comparison of specimens, 
to be certain that my determinations of doubtful species agree with those of the best 
European authorities. 
I am indebted also for specimens of RattuUdce from Lake Bologoe in Russia to 
the kindness of Dr. Romuald Minkiewicz, of the University of Kasan, Russia; 
to Herr Max Voigt, of Plon, I am under obligations for specimens of his new 
species, Diurella rousseleti. For notes and other assistance I am indebted to Herr 
Oberforster L. Bilfinger, of Stuttgart, Germany; to Prof. Dr. Otto Zacharias, 
director of the Freshwater Biological Station at Plon, Germany, and to Prof. Dr. 
Karl Eckstein, Eberswalde, Germany. It is -a pleasure to express here my thanks 
to these gentlemen. 
