154 MOLLUSC A. 



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 CYCLOSTOMA. 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 HELIX 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 



