154 MOLLUSCA. 
The mouth is furnished above with a thin-arched corneous 
mandible, notched on the edges. The whole body, includ- 
ing the foot and head, are, in general, capable of being with- 
drawn into the cavity of the shell. In two genera the aper- 
ture is closed by a lid. 
Genus CycLtostoma.—Aperture of the shell circular. 
The tentacula are linear and subretractile. The primary 
ones have subglobular, highly-polished extremities, con- 
sidered by Montagu as the eyes. The true eyes, however, 
are placed at the exterior base of the large tentacula, and ~ 
are elevated on tubercles, which are the rudiments of the 
second pair. The aperture of the pulmonary cavity is situ- 
ated on the neck. The sexes are likewise separate; the 
penis of the male being large, flat, and muscular. The 
mouth is formed into a kind of proboscis, and the upper lip 
is deeply emarginate. 
Genus Hextrx.—Snail. Aperture of the shell lunulat- 
ed; the width and length nearly equal. The snails differ 
from the slugs chiefly in the organs of reproduction. The 
vagina, previous to its termination in the sexual cavity, is 
joined by the canal of the vesicle, and by two ducts, each 
proceeding from a bundle of multifid vesicles. Each bundle 
consists of a stem or duct, and numerous branches, with blunt 
terminations. These organs secrete a thin milky fluid, the 
use of which is unknown. 
Connected with the sexual cavity is the bag in which the 
darts are produced. The bag itself is muscular, with lon- 
gitudinal grooves, and a glandular body at the extremity. 
This glandular body secretes the dart, which is in the form 
of a lengthened pyramid, consisting of calcareous filaments 
nearly resembling asbestus. Previous to the sexual union, 
the two snails touch each other repeatedly with the mouth 
