NO. 3668 PTERODRILUS—HOLT 35 
The prostate was a small gland, about % the length of the spermi- 
ducal gland, that arose about \ the latter’s length from its junction 
with the ejaculatory duct. The prostate of the more advanced species 
of Cambarincola and Pteredrilus is differentiated. In pro-pterodrilus 
it was undifferentiated and consisted of a lobe of glandular epithelium 
that was histologically indistinguishable from that of the spermiducal 
gland. There may have been a prostatic ‘‘bulb” at the ental end that 
consisted of a few differentiated cells. In more advanced species of 
Cambarincola, the bulb is a distinctive and specialized part of the 
prostate. In all species of Pterodrilus the lumen of the prostate opens 
into that of the spermiducal gland some distance entad to the junction 
of the latter with the ejaculatory duct. In Cambarincola, the prostate 
and spermiducal gland usually open together into the ejaculatory 
duct. That the prostatic glands of the two genera are homologous 
cannot be doubted, but that of Pterodrilus is closer in this respect 
than is Cambarincola to Ceratodrilus Hall (Holt, 1960a, p. 57), Elliso- 
drilus Holt (1960b, p. 172), and Oedipodrilus Holt (1967a, p. 58). 
The latter genera must on this account and others be considered as 
primitive relatives of Cambarincola. The histological differentiation 
of the prostate occurs in the more advanced species of both Cam- 
barincola (Hoffman, 1968, pp. 287, 301, et seq.) and Pterodriius 
(only P. missouriensis has a completely undifferentiated prostate). 
The evolutionary trend in the specialization of the prostate seems 
to be clear. 
The ejaculatory duct was probably short; though this supposition 
is based upon the length of the ejaculatory duct of P. mezicanus, it 
is strengthened by the fact that in the presumably primitive genera 
of the branchiobdellids, the ejaculatory duct is absent or short (Holt, 
1968). 
The bursa of pro-pterodrilus was proportionally larger than that 
found in Cambarincola and all the pterodrilids except P. missouriensis 
and P. mexicanus. The penial sheath region of the bursa may not 
have been unusually large, but the penis may have been partially 
eversible. This conjecture is based upon the opinion (Holt, 1968) 
that the primitive members of the lineage leading to Cambarincola 
and Pterodrilus possessed an eversible as opposed to a protrusible 
penis. The known members of this lineage (Magmatodrilus Holt, 
1967b, and an unnamed Mexican genus), which lack a prostate, 
possess a bursa with a large penial sheath enclosing an eversible or 
semi-eversible penis; those (Oedipodrilus and Ceratodriius) with 
incompletely separated prostates likewise have large bursae with 
eversible penes. Arguments based on the spacial relationships of the 
set of tubes that is the male reproductive system of the branchiobdel- 
lids and the conditions in other annelids have been set forth elsewhere 
