MOLLUSCA. 457 



it becomes thick, and bordered with a ring, after which 

 there is no increase of size. 



1. Last formed Whorl of the Shell greatly larger titan 

 the penultimate one. 



21. Heliv. — Snail. Aperture of the shell lunulated ; 

 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 nu- 

 merous branches, with blunt terminations. These organs 

 secrete a thin milky fluid, the use of which is unknown. 



Connected with the sexual cavity is the b^g in which the 

 darts are produced. The bag itself is muscular, Avith 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 

 and tentacula, and at last the dart of the one is pushed 

 forth by its muscular bag, and directed against the body of 

 the other, into which it enters, never penetrating through 

 the integuments, and even, in many cases, falling short of 

 its mark. Whether the use of the dart is merely to stimu- 

 late, or whether it is subservient to any other purpose, can 

 scarcely be said to be determined. 



The species belonging to this genus are numerous, and 

 exhibit, in the form, the markings, and the coverings of the 

 shell, numerous characters for their subdivision. 



2Pi. BuLi:.irs. — Aperture of the shell longer than broad. 



The structure of the animals of this genus has not been 

 determined ; but analogy would lead us to conclude, that it 



