No. 3668 PTERODRILUS—HOLT 9 
The dorsal ridges, which may bear projections in P. alcicornus 
and other species of Pterodrilus and may occur without projections in 
species of other genera, are formed by the attachment of muscles 
(‘supernumerary muscles,” Holt, 1960b, pp. 171-172) that are 
shorter than the segment to the cuticle and that, by their contraction, 
differentially shorten the dorsal surface of the prosomites in which 
they occur. There has been no suggestion by anyone, nor is it indicated 
in their structure, as to the function of the dorsal projections. They 
consist of flat ‘“‘wing-like” or cylindrical ‘‘finger-like” (prongs) exten- 
sions of the epidermis covering the dorsal ridges. The wall of the 
projections is a single-cell-layer thick (as is the epidermis) with 
unicellular glands as a prominent feature. The interior of the pro- 
jections is an irregular cavity that does not appear to communicate 
with the coelom. The finger-like projections are usually set off by a 
slight constriction. Dorsal ridges that are present on segments VI 
and VII do not bear projections on these reproductive segments in 
any species of the genus. 
Since the jaws of all species of Pterodrilus are monotonously similar, 
they have not been illustrated for all the species included in this 
study (see, however, figs. 2, 4, 5, 7). They are small, delicate and 
light yellowish brown (but see p. 11, below). The upper jaw bears 
five sharply pointed teeth, the lower four (dental formula 5/4); and 
they are more nearly quadrate in shape than is usual among species 
of Cambarincola with the same dental formula. Moore (1895a, p. 425) 
believed both jaws of P. alcicornus to be quadridentate, but this 
was probably because of the smallness of the lateral teeth of the 
upper jaw, which may cause one of them to be overlooked. 
All branchiobdellids possess two pairs of nephridia and the anterior 
pair may open by either separate pores or by a common pore on the 
dorsum of segment III. The anterior nephridia of Pterodrilus and 
related genera open by a common pore, which in P. alcicornus is 
located at the base anteriorly of the dorsal projection. The nephridio- 
pore usually cannot be seen in animals mounted entire, but there is 
little doubt that this arrangement is consistently present in Pterodrilus. 
The innermost parts of the branchiobdellid male reproductive 
system consist of testes in segment V or in segments V and VI, a pair 
of sperm funnels and sperm ducts (vasa efferentia) in each testicular 
segment, and a vas deferens from each of these segments that is 
formed by the union of the sperm ducts. These elements are quite 
similar throughout the order and will not be described here (but see 
Moore, 1895b, pp. 519-521; Holt, 1949, pp. 538-541, 550-552). 
The spermiducal gland is formed by the union of the vasa deferentia 
and does vary in shape and structure. Other than its peritoneal 
