28 PROCEEDINGS OF THE NATIONAL MUSEUM VOL. 126 
Erymo.tocy.—From Greek, choritos, native, and namos, spring or 
stream, native spring, for the spring branch of my boyhood home. 
Description.—Pterodrilus choritonamus is a small and delicate 
worm about 1.1 to 1.5 mm long. In body proportions and outline it is 
similar to P. hobbsi and P. mexicanus, differing in the absence of 
projections on the dorsal ridge of segment VIII. 
The bursa approaches half the body diameter in length and is 
pyriform in shape: the penial sheath region is set off externally by a 
slight constriction and is less in diameter than the atrial portion of 
the bursa. 
The spermatheca has the midlength of the organ (the spermathecal 
bulb) normally expanded, the ental portion not, so that there is an 
ental process that is lined with a columnar epithelium instead of a 
thin layer of flattened cells as is the bulb. Although the total length 
of the spermatheca is subequal to the body diameter, it is not as 
long as that of the other species of the genus except that of P. mezi- 
canus, which it exceeds in length. 
VaRIATIONS.—The prostate varies in length, the ental end usually 
approaching the ental end of the spermiducal gland, but sometimes 
not. The spermatheca varies in the degree of the distension of the 
bulbular region, with the result that the extent of the ental process is 
reduced by a greater expansion of the bulb, but in the specimens I 
have seen the process is present and may, then, be a constant feature 
of the species. 
Arrinities.—Pteordrilus choritonamus is related to P. missouriensis, 
P. cedrus, P. mexicanus, and P. hobbsi. Its affinities with the first 
three of these species have been discussed (p. 16). It differs from P. 
hobbsi in the absence of projections on the dorsal ridge of segment 
VIII, in the larger bursa and in having an ental process of the 
spermatheca. 
Hosts.—The known crayfish hosts of P. choritonamus are Cambarus 
tenebrosus Hay, C. extraneus Hagen, Orconectes placidus (Hagen) and 
Orconectes species. 
Disrrisution.—Pterodrilus choritonamus frequents tributaries of 
the Cumberland River in the Eastern Highland Rim in Tennessee. 
MATERIAL EXAMINED.—Types and 28 specimens. The material 
for the most part is deposited in the United States National Museum 
(USNM 36473-36476). 
Pterodrilus missouriensis, new species 
Figures 8, 9 
TyYPE-SPECIMEN.—Holotype, USNM 36469, two paratypes, USNM 
36470, and two paratypes, PCH 1476, from Orconectes luteus (Creaser) 
