20 PROCEEDINGS OF THE NATIONAL MUSEUM VOL, 125 
The spermatheca is quite similar to that of P. distichus, perhaps 
slightly broader along the midlength of the bulb. It is approximately 
as long as the body diameter and bends dorsad over the gut. The 
ental end is composed of larger cells and in many specimens there is 
a, small ental process. 
VARIATION.—The prongs of the dorsal projections vary in length, 
but this may be related in part to differences in the degree of con- 
traction. The reproductive organs show only minor differences in 
apparent shape and proportions—except for the prostate, which 
appears to be variable in length, and the extent of vacuolation. The 
ectal portion near its junction with the spermiducal gland is often 
not differentiated, but in all individuals the blind end is vacuolated 
to a greater extent and more consistently than in P. mezxicanus. 
Arrinities.—Pterodrilus hobbsi is similar to P. choritonamus and 
P. mexicanus in the absence of dorsal ridges on all segments except 
the eighth, but it differs from both of these species in the smaller 
bursa and from P. mezxicanus in the presence of five instead of four 
prongs of the dorsal projections and in the consistently differentiated 
prostate (see p. 16). The reproductive systems are most like those of 
P. distichus and P. alcicornus, species with dorsal projections on 
several segments. 
Hosts.—Pterodrilus hobbsi has been taken in association with 
17 species and subspecies of Cambarus and 4 species of Orconectes: 
Cambarus tenebrosus Hay, C. longulus longirostris Faxon, C. parvoculus 
Hobbs and Shoup, C. longulus chasmodactylus James, C. robustus 
Girard, C. veteranus Faxon, C. friaufi Hobbs, C. extraneus Hagen, 
C. bartonii cavatus Hay, C. sciotensis Rhoades, C. distans Rhoades, 
C. bartonii bartonii (Fabricius), C. longulus longulus Girard, C. 
latimanus (LeConte), C. striatus Hay, Cambarus species, C. barton 
subspecies; Orconectes erichsonianus (Faxon), O. juvenilis (Hagen), 
O. rusticus forceps (Faxon), and Orconectes species. The most common 
hosts were Cambarus longulus longirostris, C. bartonii cavatus, and 
Orconectes juvenilis. 
DistrisutTion.—Fterodrilus hobbsi inhabits most of the upper 
Tennessee drainage system and is especially common in tributaries 
of the Nolichucky, Watauga, Holston, Powell, and Clinch Rivers. 
It has also invaded the New River in Bland and Carroll Counties, 
Va., and Alleghany County, N.C.; the Big Sandy in Dickenson 
County, Va.; and is at home in a wide stretch of the Cumberland 
River drainage in Tennessee and Kentucky (fig. 9). 
MATERIAL EXAMINED.—Types and over 300 specimens from 62 
collections have been examined. The major part of this material is 
deposited in the U.S. National Museum (USNM 36488-36508). 
