8 ILLINOIS BIOLOGICAL MONOGRAPHS [128 



Camerano, but is founded on much more essential characters. That the 

 genus has found universal acceptance is perhaps due more to Montgomery's 

 characterization than to that of Camerano. The genus Parachordodes has 

 not been universally accepted; but here again the evidence presented in this 

 report shows that the characters given by Camerano are accompanied by 

 others which indicate a natural division of the group. He characterizes 

 Gordius by the presence of a postcloacal ridge in the male and the absence of 

 true areoles on the cuticula, Parachordodes by the absence of postcloacal 

 ridge and the presence of areoles. 



In America several species were described by Leidy, but most of the 

 systematic work was done later by Montgomery. Here as well as in Europe 

 little more than pioneer work has been done. Descriptions and identi- 

 fications have been made chiefly from preserved material and isolated 

 specimens and only in a few cases from living material collected in abun- 

 dance. 



The confusion that still exists in the group is due in a large measure to 

 the fact that the variations within a species are very great while the differ- 

 ences between species are relatively small. When isolated and often poorly 

 preserved specimens are studied it is natural that essential characters are 

 often overlooked and variations are taken for specific characters. This 

 tends on the one hand to throw species together and on the other to separate 

 members of a single species. 



The two characters causing the most confusion are size and color. 

 Nearly all of von Linstow's descriptions include besides these only those 

 that are common to nearly aU Gordiacea. Such descriptions are useless. 

 I have in my own collection specimens of a single species ranging in length 

 from 10 to 50 cm. and in color from light brown to nearly black and others 

 that are an iridescent gray. Even as late as 1910 Wesenberg-Lund identi- 

 fied specimens as Gordius aquaticus on account of their size and made the 

 errors that I shall point out later. 



The light spots in the cuticula and the postcloacal ridge in the male have 

 been taken as specific characters, and the species bearing either has usually 

 without hesitation been assigned to Gordius aquaticus. The ridge certainly 

 is possessed by more than one species and may be a generic character while 

 the white spots, as Montgomery suspected, are due to physiological con- 

 ditions in the American species but may be due to structures in the hypo- 

 derm in some of the European species. 



Altho the structure of the cuticula is one of the best specific characters, 

 its use has led to confusion because different authors have studied it under 

 different conditions and have made different interpretations of what they 

 saw. The erroneous theory that size, color and cuticular structures change 

 with the age of the free living specimen has also contributed to the tangle. 



