3214. With the development of the head or rather of its inferior organs of sense, the antagonism makes its appearance also in the ovarium. A half of the ovarium is converted into the testis.
The Snail is therefore a Mollusc, which is female upon one side, male upon the other.
3215. The androgynous or bisexual animal is, as a general rule, asymmetrical.
3216. The mantle also or the branchial cavity obeys this want of symmetry. The branchiæ of one side dwindle down; those of the other turn with the mantle towards the head, and the respiratory aperture occurs upon the back.
3217. With the one-sided evolution of the mantle, one shell also is only developed, while the other is stunted or placed under arrest. The Snail's shell is one of the Bivalve Mollusc's shells, its operculum is the other. This last is stony, horny, and finally is entirely wanting.
It is remarkable that the right shell has been pretty generally perfected, while the left dwindles down into the operculum; all the Snail's openings are therefore upon the right side, such as the anus, with the orifices for the escape of the ova and semen.
Male animals are situated on the right side, female on the left; or where the right side was predominating, there originated the male sex, where the left, the female.
3218. As the orifice of the mantle and the shell is properly the aperture of the branchial foramina; so can it be said, that the Snail were a Mussel, which does not simply extend the foot, but also the mouth or head to the branchial opening; it is a Mussel reversed.
3219. Every thing else in the Snails is arranged in accordance with these fundamental organs and forms.
The cephalic portion of the intestinal canal is indicated also by muscular fibres. The pharynx and the mouth can contract and expand, seize and bite off; the former frequently admits of being protruded and retracted as a fleshy proboscis with perforating maxillæ.