SPECIAL ANATOMY. 227 
is strongly muscular, and internally presents a number 
of regular, longitudinal folds, sometimes undulated at the 
sides, extending to the lining of the bladder in the form 
of line-like plicre. In H. ligera, H. intertexta, IT. 
gularis, and H. suppressa, an offset from the duct of the 
bladder passes down, and encloses the penis, dart sac, 
and cloaca. 
The vagina, or common duct of the oviduct and duct 
of the genital bladder, holds no correspondence with the 
length of the penis ; it is always shorter, usually not 
more than one-third the length, and is also narrower. 
In M. fuliginosa, it is surrounded by a thick, glandulous 
body. 
In H. ligera, H. intertexta, H. gularis, and H. sup- 
pressa, there exists, opening into the cloaca, a curved, 
cylindrical, strongly muscular dart sac, longer and nar- 
rower than the penis. The bottom of the tube, for one- 
fourth the length of the latter, is occupied by the papilla 
from which arises the dart. The muscular layer, for 
more than half the length of the tube, at the middle of the 
latter, closely envelops the dart, and terminates abruptly 
below in a sort of papilla, from which the point of the dart 
projects into the lower part of the tube. The dart is a very 
long, narrow, curved, cylindrical, tubular, flexible, calca- 
reous spiculum, terminating in a sharp, spear point. At 
the base of the dart, there opens into the dart sac, in H. 
ligera and H. suppressa, a single, short, pyriform follicle, 
the simplest homologue of the multifid vesicle. In M. in- 
tertexta and H. gularis, there is a pair of such follicles, 
