10 PROCEEDINGS OF THE NATIONAL MUSEUM VOL. 125 
covering and a very thin layer of circular muscles, the gland consists 
of a tube composed of a single layer of columnar epithelium, the 
individual cells of which open into a narrow lumen. All of these gland 
cells are filled with granules though differences in the secretory cycle 
of individual cells can be detected in sectioned material. There are 
no apparent differences in the histological appearance of the spermi- 
ducal gland of P. alcicornus and the other species of Pterodrilus and 
that previously described for a species of Cambarincola (Holt, 1949, 
p. 552). 
The prostate is a glandular diverticulum of the spermiducal gland 
that lies along the anteriodorsal border of the latter, ends blindly 
entally and opens either into the spermiducal gland somewhere along 
its ectal portion or with it into the ejaculatory duct. In P. alcicornus 
and the other species of Pterodrilus, the prostate opens into the sper- 
miducal gland somewhat entally to the junction with the ejaculatory 
duct; that is, it is incompletely divided from the former. The prostate 
is of the same basic structure as the spermiducal gland from which it 
arises, but in P. alcicornus its glandular epithelium consists of highly 
vacuolated cells with different staining properties from those of the 
spermiducal gland. This histological difference between the two glands 
is readily apparent in well-prepared whole mounts and is present in 
all mature individuals of species of Pterodrilus with the exceptions 
mentioned below (pp. 16, 25). The expressions ‘differentiated’ and 
“undifferentiated” are used to distinguish between such prostates as 
those of P. aleicornus and those that are histologically lke the sper- 
miducal gland in both sectioned material and whole mounts. In many 
species of the genus Cambarincola, the ental end of the prostate consists 
of a thin-walled bulb, the interior of which is a cavity (Holt, 1949, 
p. 553; 1960a, p. 63). There is no prostatic bulb in P. alcicornus. 
The ejaculatory duct is a muscular tube which is found in most 
branchiobdellids and unites, if present, the spermiducal gland and the 
copulatory bursa. That of P. alcicornus is not unlike that of other 
species of the genus except in length. 
The copulatory bursa of P. alcicornus and other species of the genus 
is quite similar to that of members of the genus Cambarincola. The 
penial sheath region of the bursa in P. alcicornus is not demarcated 
externally from the bursal atrium and composes about half of the 
organ. When the atrial portion of the bursa is everted, the penis is 
protruded as a short and relatively slender tube surrounded by the 
cuplike everted bursal atrium. More detailed descriptions of the type 
of bursa found in the species of Pterodrilus may be found in Holt 
(1949, pp. 553-555) and Hoffman (1963, pp. 289-290). 
The ovaries and ovipores of all branchiobdellids seem to be basically 
similar (Moore, 1895b, pp. 524-525; Holt, 1949, pp. 545-547, 560). 
