The families of British non-marine molluscs (slugs, snails and mussels)
Morphology. Snails, with a conspicuous, spiral, univalve shell.
The animal with one pair of tentacles only. Eyes at the bases of the tentacles.
The shell inoperculate; planispiral (nearly always), or rising-spiral (in Gyraulus crista only); 3–8 whorled; typically sinistral (when correctly interpreted, although carried by the animal upside down and hence appearing dextral when illustrated in its natural position. The morphologically upper side, as determined by the anatomy, is that which is uppermost when the shell is held with its aperture to the left of the observer); wider than high; 2–17 mm wide, or 25–35 mm wide (Planorbarius corneus); height about (0.15–)0.2–0.36(–0.44) x the width. The height of the spire about 0.12 x that of the shell (in the discoid Gyraulus crista, the rest being planispiral). The spire obtuse. The shell in Gyraulus crista, where the spire is raised, discoid. The whorls neither shouldered nor keeled, or shouldered, or keeled; conspicuously and regularly transversely ridged across the whorls (notably in Gyraulus crista, and in the Nautilus-like, internally segmented Segmentina nitida), or not conspicuously transversely ridged (then sometimes conspicuously ridged at irregular intervals). The aperture with neither teeth nor calluses. The shell with an umbilicus. The umbilicus large and wide. The shell thick-lipped to thin-lipped; thin and translucent to opaque; yellowish, brownish or olivaceous horn-coloured, occasionally whitish, sometimes more or less translucent, but often darkened by extraneous matter so as to appear blackish; plain. Morphological comments. Unlike that of most molluscs, the blood contains haemoglobin, so that a popular aquarium form of P. corneus with a translucent shell and lacking body pigment is bright red.
General biology, ecology. Freshwater aquatic. Breathing air directly via the lung-like mantle cavity (and via the skin in general). Water snails of diverse, well-vegetated aquatic habitats, with some species tolerant of seasonal dessication.
Hermaphrodite (but individuals acting as either male or female when mating).
Classification. Gastropoda; Pulmonata.
British representation. Rams-horn snails: Anisus (3), Bathyomphalus (1), Gyraulus (4), Hippeutis (1), Menetus (1), Planorbarius (1), Planorbis (2), Segmentina (1).
Illustrations. • Anisus, Bathyomphalus, Gyraulus, Menetus, Planorbarius, Planorbis and Segmentina (with Physidae: Adams). • Anisus, Bathyomphalus, Gyraulus, Hippeutis, Planorbarius, Planorbis, Segmentina (Reeve). • Anisus, Bathyomphalus, Gyraulus, Hippeutis, Menetus, Planorbarius, Planorbis, Segmentina (Ellis). • Planorbis corneus (J.A.).
To view the illustrations with detailed captions, go to the interactive key. This also offers full and partial descriptions, diagnostic descriptions, differences and similarities between taxa, lists of taxa exhibiting or lacking specified attributes, and distributions of character states within any set of taxa.
Cite this publication as: ‘Watson, L., and Dallwitz, M.J. 2005 onwards. The families of British non-marine molluscs (slugs, snails and mussels). Version: 4th January 2012. http://delta-intkey.com’.