NOTES ON SOME FREE-SWIMMING ROTIFERS., 145 
perfectly safe inferences respecting its morphologic constitution. On 
the other hand, aM minute structures whose elements lie so close 
together as to occasion noticeable diffraction phenomena will not be 
geometrically imaged, that is to say, the image will not be formed, 
point for point, as usually described by the reunion in a focal 
point of pencils of light which, starting from the object, undergo 
various changes of direction in their entrance and passage through 
the objective. 
(Zo be continued. ) 
NOTES ON SOME FREE-SWIMMING ROTIFERS. 
BY; je. Lob. 
N my last contribution I spoke of the difficulty the student 
constantly experiences in his endeavour to classify some of the 
less-frequently recurring forms of the Rotifera. This difficulty is 
materially increased by the loose and irregular manner in which 
the different species are described. Sometimes this description 
will commence with the head and sometimes not. In some cases 
we have an elaborate description of the mastax (not always correct), 
while in others this important organ is not even mentioned. Occa- 
sionally the general configuration is given very fully, in precise 
language ; in other cases, if mentioned at all, only in such vague 
and general terms, as to be of little or no service. Such difficulties 
as these are very perplexing to the earnest student, and might easily 
be remedied if the authorities would only exercise a little more 
care. It appears to me, that if a certain order was maintained, in 
the description of every animal, it would render the comparison of 
kindred forms a much more easy task. In this paper I propose to 
figure and describe several free-swimming Rotifers, which have 
puzzled me, one of which, at least, I think will be new to science. 
Fig. 21. This Rotifer I have procured in tolerable numbers from 
a well, during the whole of last summer. It is of small size and 
loricated. In most of its characters it agrees with the Genus 
Monostyla, but I am far from being satisfied that it agrees with it 
in that particular point from which the Genus takes its name. 
Pritchard says of this Genus, ‘‘ Owing to the almost constant vibra- 
tion of its foot-like tail, it is difficult to observe the true form of its 
termination, the motion producing an optical deception; hence it 
appears double though really single.” In spite of this “ warning 
note,” I believe that the tail-feet of the Rotifers, Figs. 21 and 22, 
are furcate, though seldom separated. I ought perhaps to say, 
