I have observed that individuals of Limnæa auricularia can continue to propagate for several generations without being impregnated; they must impregnate themselves.
Order 2. Conchozooid Snails—Diœcii.
3497. Branchiæ situated within the cavity of the mantle, and hanging down in the form of a comb; shell for the most part spirally contorted; sexes separate—Pectinibranchiata.
Here belong the Capulidæ, Turbinidæ, Neritæ, Conidæ, Volutidæ, Buccinidæ, Muricidæ and Strombusidæ.
The tentacula are not retractile, and have the eyes mostly seated at their basis; the penis is external, very large, and cannot be drawn within the body, but is only reflected into the pallial cavity; most of them have a protrusile perforating proboscis, and an operculum. They lay numerous small ova, contained within large membranous cases, which frequently hang in rows to each other like a necklace of pearls. The shells are in some instances horny, in others of a stony texture.
Fam. 4. Mussel-Snails, Capulidæ.
Only one branchial comb within the mantle, and covered merely by a flat hood-shaped shell; no operculum.
Here belong the Capulidæ; all the species are marine. They prefigurate the Brachiopoda.
Fam. 5. Typical Snails, Turbinidæ.
Two branchial combs, mantle devoid of a respiratory groove, shell turbinated; mostly furnished with an operculum, and that indeed of a stony texture. Here belong the Turbinidæ, and Trochidæ, such as the Cyclostomata, Paludinæ, Ampullariæ, Janthinæ, Neritæ. Live in the sea and in fresh-water.