NO. 3668 PTERODRILUS—HOLT rf 
Description.—The length of individuals of Pterodrilus alcicornus 
(based on 10 specimens) is about 1.3 mm. The head is slender, its 
length about % that of the body and its diameter about ¥% that of the 
greatest body diameter. The intersegmental grooves of the head, 
except for that setting off the peristomium, are indistinct. 
The trunk or body is cylindrical throughout and increases grad- 
ually in diameter up to the reproductive segments (V—-VII), which 
are all essentially the same diameter (about 0.25 mm). The sucker, 
formed from segment XI, is subequal to or slightly greater than the 
head in diameter. 
The dorsal appendages or projections are borne on ridges of the 
prosomites. In P. alcicornus, they are paired lobes that diverge some- 
what and extend laterally and anteriorly in the case of the anterior 
three to form forward-facing concavities. The projection of segment 
VIIT is similar, except that it faces posteriorly. The lobes (‘‘wing- 
like”? projections) bear conical prongs, usually three on each side, 
although the number varies from one to four prominent prongs, with 
smaller ones frequently present. The lobes of the projection of seg- 
ment V do not flare out quite so much as do those of the others and 
the prongs project more nearly upward. Dorsal ridges are present on 
the prosomites of segments VI and VII. 
The spermiducal gland is thick for its length, with a length- 
diameter ratio of about 3:1. In length and diameter, the prostate is 
about *4 of these dimensions of the spermiducal gland though it often 
appears in whole mounts to be subequal in diameter to that of the 
latter. The prostate is differentiated. The ejaculatory duci is about 
% the length of the bursa and slightly expanded along its midlength. 
The diameter of the bursa is approximately * its length. The bursal 
glands mentioned by Moore (1895a, p. 454) are not present. He 
described as glands the fold of the wall of the bursal atrium that 
becomes the ‘“‘rim of the cup” of the everted bursa. 
The ectal duct of the spermatheca is long and slender, widening 
gradually into the bulb, which is also long and bends over the gut 
dorsally; the total length of the spermatheca exceeds the body 
diameter. There is no ental process of the spermatheca though the 
ental end of the bulb may resemble such a process when incom- 
pletely filled with spermatozoa. 
Discussion.—The following account of the anatomy of P. alcicornus 
is a condensation of my unpublished earlier treatment (Holt, 1951, 
pp. 101-115). Serial sections were used as well as whole mounts and 
the earlier observations confirmed by more recent examination of 
many specimens. The abundance of this material affords an oppor- 
tunity to describe P. alcicornus is some detail and, thereby, present 
