162 



is probably a polycyclic planktont with its greater pulses in spring 

 and fall. 



' Asplanchnopus myrmcleo 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 Ploima), 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 Brachionus 

 problem from the statistical standpoint is made difficult by the 

 coiidition of the literature of the subject, owing largely to the semi- 

 tropical distribution of the genus; by the absence of any critical 

 monograph of the whole genus dealing fully with the synonymy 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 



