Gastropoda, in which the primitive bilateral symmetry is retained in the head and foot, whilst the visceral dome with the mantle fold is twisted from behind round the right side more or less to the front, so that the ctenidia, anus, nephridial and generative pores are placed either on the right side of the body or 'more or less anteriorly, the primitive right ctenidium thus appearing on the left side of the anus or body. The visceral dome frequently continues to grow with a similar but spiral twist, and its surface; with the mantle, is generally protected by a more or less spiral shell. The primitive left ctenidium and left (? right) nephridium 1 usually atrophy. The genital gland and duct are single. A superficial secondary bilateral symmetry is sometimes acquired (Natantia, Opisthobranchia). The foot has commonly a pedal gland.
The foot is generally broad and flat, and sometimes much expanded. It is divided into a distinct pro- and meso-podium, and a metapodium in many Reptant Azygobranchia and all Natantia. The pro- and mesopodium of the latter group are flattened laterally, and the mesopodium frequently carries a sucker, whilst the metapodium forms, as a rule, a large swimming tail. The metapodium often bears upon its dorsal surface in many Azygobranchia, even when not divided off from the mesopodium, a spiral and calcareous, or chitinoid operculum, which closes the aperture of the shell when the animal is completely retracted within it. This operculum is sometimes present in the embryo Opisthobranch, but is always lost in the adult. Lateral lobes of the foot, extending for a greater or less extent along its sides, and known as epipodia, are present in some Azygobranchia and many palliate and a few non-palliate Opisthobranchia; and the cephalic hood or fold above the head, seen in many members of the last-named group, is perhaps an anterior extension of these folds. The foot is completely aborted in some of the non-palliate Opisthobranchia, e. g.
Phyllirhoe. The visceral dome is not distinctly marked off in the order just named and in some Pulmonata. Its torsion is sometimes 'reversed/ and certain genera or species, e. g. the Whelk (Buccinum undatum) often show a great tendency to this reversal. The dome varies much in size and development, and is most marked in the Reptant Azygobranchia and some stylommatophorous Pnlmonata. So too with the mantle fold. It may be aborted altogether in the adult of some Opisthobranchia, hence termed Non-Palliata, e. g. Doris, Eolis, Phyllirhoe, or become insignificant in size, as in some Pulmonata (Limacidae). It generally forms a low ridge, but is well developed when it incloses the ctenidia in a branchial cavity, which opens either to the right side or anteriorly. The aperture of this branchial cavity in most Pulmonata is reduced to a small contractile pore by the concrescence of the free edge of the mantle with the body wall. The edge of the mantle is often produced into variously-shaped processes, and is sometimes reflected over the edges of the shell when the animal is fully expanded temporarily, as in many Streptoneura, or permanently as in Aplysia among palliate Opisthobranchia.
1 So says Professor Lankester; but recent researches make it probable that the primitive right and not the left, nephridium atrophies. See p. 479 and note. The word 'primitive' implies the organs of the right and left sides before the occurrence of any spiral twist.
The primitive shell-sac of the embryo appears to persist in some Pulmonata, and incloses in Arion a mass of crystals of Calcium carbonate, or a small shell as in Limax. The shell of Clausilia is also stated to originate within it in the first instance. But as a rule it disappears, or only contributes a minute portion to the shell, which is formed by the edges and surface of the mantle and visceral dome. The size of the shell is therefore correlated with the size of these two regions. Its shape is variable: sometimes plate-like, e. g. Aplysia; conical, as in the Limpets Fissurella and Patella; usually spiral. The axis of this spiral may be steep, or at various degrees of obliquity: almost horizontal, forming a disc-like shell in Planorbis. In some cases successive coils inclose one another more or less completely, e. g. Conus or Cypraea. The spiral twist follows that of the visceral dome, and is therefore usually right-handed or dextral: and it is rare for a lefthanded or sinistral twist to be normal in a given genus or species. The successive coils of the spiral are closely applied to one another as a rule, but the lower coils may become loose and straggling, e. g.
Siliquaria, Vermetus; and in Magilus, when the animal becomes inclosed by the corals among which it lives, the shell may lose its spiral growth altogether. The adult of the Non-Palliate Opisthobranchia is devoid of any shell at all. The Veliger, however, possesses a small nautiloid shell, which is discarded during growth, and the same is true of the shell-less Azygobranchia Natantia. The first formed shell aborts in some cases, and a second shell, unlike the first, is then formed, e.g. Marsenia among Reptant Azygobranchia. The texture of the shell varies. It is often brightly coloured, and the coloration is derived from the glands at the edge of the mantle. Its anterior edge is sometimes produced into a spout-like notch, along and beyond which a grooved process of the mantle is capable of protrusion. This process is the siphon, and is characteristic of the carnivorous sub-order Siphonochlamyda of Azygobranchia Reptantia, e. g. the Whelk.
The ectoderm, or epidermis, consists of a columnar epithelium provided with a delicate cuticula, and which, where it is exposed, is often ciliated to a greater or less extent. It contains unicellular mucous glands, pigment glands, and glands the secretion of which is loaded with Calcium carbonate. The two latter kinds of glands are especially numerous on the edge of the mantle, where they secrete the colouring matter of the shell, its outer epicuticula, and the greater part of its calcareous substance. But the cells of the surface of the mantle and visceral dome also have the power of repairing injuries to the parts of the shell already formed. The mucous glands are sometimes large, and then lie in the sub-epidermic connective tissue. A series of transverse lamellae is developed in many Streptoneura to the right of the ctenidium on the roof of the branchial cavity. They constitute the hypobranchial gland, and secrete much mucus. In the genera Purpura and Murex the secretion, at first colourless, changes in sunlight to a purple or violet, used as a dye by the ancients, and known as 'Tyrian purple' Specialised glands are found in connection with the foot, and may be distinguished into a suprapedal gland, which opens between the head and foot, e.g. Helix, Succinea among Pulmonata, Vermetus, Cyclostoma among Azygobranchia Reptantia; and a pedal gland, which opens on the surface of the foot itself, as in certain of the latter group, e. g.