14 PROCEEDINGS OF THE NATIONAL MUSEUM VOL. 125 
dorsal projections, that of segment VIII with five projections; bursa 
subspherical, small, hardly reaching ventral border of gut; ejaculatory 
duct of medium length; prostate broadly joined to spermiducal gland, 
subequal in length and diameter to latter, differentiated; spermiducal 
gland small, its length about twice its diameter; spermatheca clavate, 
length subequal to body diameter, ectal duct gradually merging into 
bulb, without ental process. 
DescriIpTion.—In size the members of this species differ very little 
from those of P. alcicornus, but are perhaps slightly larger, with an 
average length of 1.4 mm. The dorsal ridges would hardly attract 
attention if they did not bear projections. The latter are relatively 
short and about io the greatest body diameter in length. There are 
two of these on segments JI-VIT and five on segment VIII. 
The spermiducal gland, ejaculatory duct, bursa, and penis differ 
from those of P. alcicornus only in their smaller size. 
The spermatheca is not proportionally as long as that of P. alcicornus 
and does not bend appreciably over the gut although its length is 
approximately equivalent to the body diameter; the ectal duct appears 
to be somewhat wider than that of P. alcicornus and is not greatly 
different in diameter from the bulb at the union of the two. 
VarriatTion.—Minor differences that may be noted in the size of 
specimens, the length of the dorsal projections, and the size and 
proportions of the reproductive systems perhaps are best correlated 
with differences in age or nutrition and in the methods of killing and 
preserving. 
ArFinitiges.—The differences between P. distichus and P. alcicornus 
have been noted (p. 12). The two species are closely related. In the 
number of dorsal appendages, P. distichus agrees with P. simondst, 
but the reproductive systems of these two species are significantly 
different (p. 25). 
Hosts.—Pierodrilus distichus has been associated with Orconectes 
propinguus (Girard), O. immunis (Hagen), O. obscurus (Hagen), O. 
juvenilis (Hagen), O. rusticus rusticus (Girard), Cambarus robustus 
Girard, C. bartonii barton (Fabricius) and C. longulus chasmodactylus 
James, of which the two most frequent hosts are O. propinguus and 
C. robustus. 
Distrripution.—Pterodrilus distichus has been taken from the states 
of New York, Ohio, Kentucky, Indiana, Illinois, and Michigan (fig. 
10). All of these records are from regions covered by ice during the 
Wisconsin glaciations except those from Breathitt, Madison, Jessa- 
mine, and Harrison Counties, Ky. The first three of these Kentucky 
records are from the Kentucky River drainage, the last from the 
Licking River system, both streams of the Ohio drainage. The con- 
clusion is that the ancestors of P. distichus have moved from some- 
