Physical characteristics

Neritopsina shell morphology covers most forms seen in the Gastropoda: coiled to limpet, and even shell-less slugs. The Neritopsina have a distinctive juvenile shell, or protoconch, and most species absorb the internal partitions of the shell as they grow, permitting the snail's body to remain more limpet-like rather than coiled. Neritopsis radula does not absorb the internal partitions of its shell and, based on this and other anatomical characters, is thought to represent the most basal taxon. The most typical coiled morphologies are seen in the terrestrial groups Helicinidae and Hydrocenidae, and are similar to those seen in coiled vetigastropod groups such as the Turbinidae. All limpet morphologies are marine groups and include both conical and coiled forms, again similar to that seen

Smaragdia viridis. (Photo by Danilo Scuderi. Reproduced by permission.)

in vetigastropods groups (e.g., Fissurelloidea and Haliotoidea). The most common marine form is a coiled limpet-like morphology with a flat shelf posterior to the aperture. This porce-lanous shelf is often ridged, forming tooth-like structures. Shell sculpture varies widely from simple concentric growth lines to taxa with heavy radial ribbing. Several freshwater species have spines that extend from the penultimate whorl, and are often covered by a dark, thick periostracum. Most species have a calcium carbonate operculum that is used to cover the aperture, and the operculum has an apophysis, or spur, on the internal surface. The animals are supple and they have a single pair of cephalic tentacles (hydrocenid species lack tentacles, but the eyes are stalked). Some deepwater and limpet-like taxa bear sensory epipodial tentacles.

The neritopsine female reproductive system is complex, with multiple openings, fertilization is internal, and a muscular copulatory structure is typically found on the right side of the head; a penis is lacking in the terrestrial groups. Spermatophores to facilitate sperm transfer are present and sperm morphology is also distinct in the group. Gelatinous egg capsules are produced and these may be further packaged into calcium carbonate-impregnated egg masses. Free-living veliger larvae are planktotrophic, but direct development is present in some freshwater species and in all terrestrial taxa. The neritopsine radula is rhipdoglossate, as in the Cocculiniformia and Vetigastropoda.

Neritopsines are generally small- to medium-sized gastropods, and range between 0.07-1.5 in (2-40 mm), a relatively small size range within the Gastropoda. External color patterns are variable from solid dark and light colors to bright greens. Shell markings are often geometrical in zigzag shapes. Terrestrial neritopsines often have bright color markings and glossy shells, while others are more subdued, mottled, and blend into their habitats well.

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