34 PROCEEDINGS OF THE NATIONAL MUSEUM VOL. 125 
greatly, if at all, exceeding 2.0 mm in length. As branchiobdellids go, 
they were graceful animals, and their size was probably an adaptation 
that enabled them to escape competition with their larger relatives 
by retreating farther into the smaller crevices found on the underside 
of a crayfish than their relatives could and there exploiting the food 
found in such crannies. 
The dorsum of the prosomite of segment VIIT on these animals was 
raised into a ridge by the existence of supernumerary muscles. Such 
ridges are found on the prosomites of one or several segments of a 
number of branchiobdellids in genera that are not closely related to 
Pterodrilus, as well as among the species of the genera Cambarincola 
and Oedipodrilus Holt (1967a, p. 58). Perhaps this arrangement of 
the body-wall musculature is related in a mechanical sense to the 
hirudinoid mode of locomotion adopted by the branchiobdellids. 
One might conclude, then, that the absence of these ridges on all 
segments other than segment VIIT is a primitive condition and that 
the evolutionary trend in Pterodrilus has been in the direction of an 
increasing number of such ridges. 
The tendency in the genus Pterodrilus for the dorsal ridges to 
bear projections of unknown adaptive significance is shared with 
Ceratodrilus and the Asian Cirrodrilus, genera that are dissimilar to 
Pterodrilus in most other respects. The primitive progenitor of Ptero- 
drilus lacked these projections, as the species P. missouriensis and 
P. choritonamus attest. In spite of our ignorance of the adaptive 
significance of these projections, it is assumed that the species with 
few or none are more primitive in this respect than are those with 
dorsal projections on several segments. 
The jaws of pro-pterodrilus were generally small and delicate in 
appearance: the upper bore five teeth; the lower, four. Except for 
the reduction in size, this is the usual, and probably primitive, pattern 
in the genus Cambarincola and that found in all species of Pterodrilus. 
The cylindrical body shape, common anterior nephridiopore and 
5/4 dental formula are features shared by Pterodrilus and Cambarincola 
and hence by the progenitor of Pterodrilus. 
The innermost parts of the male reproductive system are basically 
the same in all branchiobdellids (Holt, 1965, p. 26) and nothing needs 
to be said about the testes in segments V and VI, the efferent funnels 
and ducts, or the deferent ducts. The spermiducal gland received the 
deferent ducts entally without the deferent lobes (Hoffman, 1963, 
p. 286) that are found in some putatively primitive species of Cam- 
barincola. The gland had a lesser relative diameter and a proportion- 
ally greater length than that in all the species of today except P. 
missouriensis and, to a lesser extent, P. mexicanus. 
