650 THE AMERICAN NATURALIST. (VOL. XXXIII: 
irregular and sporadic in occurrence, so that one usually comes 
upon them accidentally. Occasionally 50 or 100 more indi- 
viduals are found forming an interlacing ball. 
As to the preservative methods, strongly penetrating fluids 
must be employed, owing to the thick cuticle encircling the 
body ; for good histological fixation it is best to use Flemming’s 
fluid, after cutting the worms into short segments. 
The following key is for the species thus far described of the 
North-American continent, including Mexico; no specimens 
are yet described from the southeastern United States, nor 
from north of the United States. We have three well-marked 
genera: Gordius Linn., Chordodes (Creplin) Mobius, and Para- 
gordius Camer. (Montg.). The females are, as a rule, more diffi- 
cult to determine than the males, and the females of Gordius 
platycephalus and G. densareolatus are often barely distinguish- 
able, while their males may be easily distinguished. This 
key enables one to readily distinguish the genus and sex, the 
species of the male individuals, and the females, except those 
of the genus Gordius, which must be determined by reference 
to more detailed descriptions. The males of Chordodes morgani 
and Gordius leidyi, and the females of G. /ongareolatus, G. 
aquaticus difficilis, and Chordodes puerilis are still unknown. 
Chordodes puerilis and C. morgani may eventually be found to 
be the two sexes of the same species. 
In the following literature list the papers by Montgomery 
contain full descriptions of the American species (with figures), 
as well as references to all previous writings upon them; and 
the other papers mentioned are the chief systematic mono- 
graphs on the group : — 
. Camerano: “ Monografia dei Gordii.” Accad. Reale delle 
si. di Torino, 1897. 
2. Montgomery: “ The Gordiacea of Certain American Col- 
lections.” Bull. Mus. Comp. Zoblogy, Harvard, Vol. XXXII, 
1898. 
3. Montgomery: Ibid., Pt. It. Proc. Cal. Arad. Sai; 32 
Ser., Vol. I, 1898. 
4. Romer: “Beitrag zur Systematik der Gordiiden.” Ad- 
handl. Senckenberg. naturforsch. Ges., Bd. XXIII. 
