Here belong the Tritoniæ and Dorides. Their body is muscular; the tentacula not retractile; the male sexual parts open in company with the female upon the right side of the neck, as in the higher organized Snails. All inhabit the sea. They prefigure or typify the Salpæ.
Fam. 2. Polypary Snails—Pleurobranchiata, or Patellæ.
Body and sexual parts as in the preceding family, the branchiæ, however, occurring as ramules or leaflets upon the sides of the body, are more or less covered.
Here belong the Phyllidiæ, Schüssel-and Schildschnecken. They are the antetypes of the Ascidiæ.
Fam. 3. Acalephan Snails—Dictyobranchiata or Limacidæ.
The branchiæ form a rete or network within the pallial cavity, and respire the moisture of air; mantle and viscera are mostly surrounded by a shell; the body is therefore bipartite, being separated into a splanchnic or visceral body and a foot with head.
Here belong the Air-breathing Snails, both land as well as fresh-water species. They typify the Cirripedia.
The shells are mostly thin and horny, yet nevertheless contain a considerable quantity of calcareous earth, and are mostly devoid of opercula.
Those species, which dwell in fresh water, do not possess introvertible tentacula like the marine Snails, and the eyes are placed at their basis; the sexual orifices are separate.
In the Land-snails, the tentacula are introvertible, and support the eyes upon their apex; the sexual apertures are blended into one. The former, like the marine Snails, lay numerous small ova inclosed within a gelatinous mass in the water; the latter deposit their ova free, covered with a membranous, and occasionally calcareous shell, in the earth. Copulation is effected in all by a reciprocal interchange of the androgynous species.