Arteriose, Orchitic Animals—Snails.
3495. The Snails likewise divide into two orders, according to the two circles, preindicated by the Ovum-and Sexual system. Their branchiæ are either ramulate or pectiniform, the sexual parts combined or separated.
The first kind are still frequently gelatinous in texture, transparent and naked; their branchiæ usually stand out freely as filaments, lamellæ, or ramules upon the back, or lie simply as a vascular network within the mantle. All are androgynous. They therefore obviously repeat the Protozoa or Mucus-animals.
The second are invariably covered by the shell and by a mantle, within the cavity of which the branchiæ lie concealed as one or two comb-like bodies. Tentacula and eyes, which are occasionally wanting in the preceding order, are here universally present; the sexes separate.
As in the Snails, the male parts make their appearance for the first time distinct and individualized, and are also a characteristic organ of the class; so also do they serve as bases of division, and the Snails may be divided into those which are Androgynous or bisexual, and those with separate sexes, i. e. Diœcious.
Order 1. Protozooid Snails—Androgyni.
3496. The male and female sexual parts united together in a single individual, branchiæ ramuliform, occurring either as filaments, leaflets or ramules, freely situate upon the naked body, or as a network lodged within the pallial cavity and surrounded by a shell.
The Nudibranchiate species live in the sea, those with hollow branchiæ or the Pulmonea in the air; the former feeding mostly upon animals, the latter upon plants.
Fam. 1. Infusorial Snails—Notobranchiata, or Tritoniæ.
Body gelatinous and membranous, cylindrical and naked, devoid of shell, with branchial filaments or ramules disposed in two rows upon the back.