162 
is probably a polycyclic planktont with its greater pulses in spring 
and fall. 
Asplanchnopus myrmeleo Ehrbg.—Taken in small numbers and 
irregularly from May to October at temperatures above 60°. 
Ascomorpha ecaudis Perty.—Found rarely in early summer, in 
temperatures above 60°. 
Brachionus. 
The discussion of the species of this genus in our plankton 
is fraught with great difficulty. The genus is represented in the 
Illinois River by a very large number of individuals (fully 25 
per cent. of the total Plowma), and the species are, almost without 
exception, exceedingly variable. They are loricate forms, and the 
variations affect the proportions of the lorica and the development 
of its prolongations in spines, antlers, and various diversifications 
of its surface. . They are evident upon the most cursory examination 
in most cases, and have been utilized by systematists for the estab- 
lishment of species. For example, Weber (98) lists no less than 67 
species of Brachionus, the most of which he regards as synonyms, and 
he includes only a part of the species. Fuller knowledge of the 
extreme variability in this genus has led the most thorough students 
of the rotifers to regard many of these so-called species as but 
varieties at the best, and to express their opinion with unmistakable 
plainness that descriptions of new species among rotifers should only 
be made after most careful determination of the variability of the 
organism (cf. Rousselet, 02, Jennings, 00, Wesenberg-Lund, ’00, 
and Weber ’98). 
For one not a specialist in rotifers, the attacking of the Brachtonus 
problem from the statistical standpoint is made difficult by the 
condition of the literature of the subject, owing largely to the sem1- 
tropical distribution of the genus; by the absence of any critical 
monograph of the whole genus dealing fully with the svnonymy of 
the subject; and by the necessity of establishing and maintaining 
constantly amid the ceaseless change of varying forms the same 
standards of distinction between the species or varieties into which 
all of the individuals enumerated must be assorted. Furthermore, 
these distinctions must be established before the plankton is 
counted; that is, before the limits of variation are fully appreciated. 
It is needless to say that my efforts are at best but approximations 
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A a he Wy = 1 

