The Families of British Non-marine Molluscs (Slugs, Snails and Mussels)

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L. Watson and M. J. Dallwitz

Character List

Morphology

#1. <Presence or absence of shell, and its constitution: slug, snail, limpet or mussel>/

1. snails, with a conspicuous, spiral, univalve shell <this serving as a shelter or retreat for the animal>/

2. limpets, with a conspicuous, protective patelliform shell bearing only a vestigial spire/

3. slug-like, but with a small, posterior external shell covering the mantle <the shell far too small to contain the animal>/

4. slugs, with no external shell/

5. mussels, enclosed by a hinged, bivalve shell <the animals lacking a developed head>/

Univalve: a shell consisting of one piece.

Bivalve: a shell composed of two plates, hinged together.

#2. <Head-to-tail length of fully grown slug>/

mm long when fully extended/

#3. <Slugs with no external shell, form of the internal shell>/

1. with a small, totally internal, symmetrical shell/

2. with a small, totally internal, asymmetrical shell/

3. with a degenerate internal shell <sometimes reduced to a few granules>/

#4. <Slug, whether relatively small and wormlike>/

1. relatively small, slender and almost worm-like when fully extended <Boettgerilla>/

2. not slender and worm-like when extended <implicit>/

#5. <Slugs, whether assuming a hemispherical or sub-globular form when molested>/

1. contracting protectively into into a hemispherical or (Geomaculus) sub-globular form when molested <Arionidae>/

2. contracting protectively when molested, but not assuming the hemispherical or sub-globular form characteristic of Arionidae/

#6. The body <of the slug, colouring>/

#7. The mucous <of slugs, colour>/

1. colourless/

2. milky/

3. yellow or orange/

4. colourless from the foot, and yellow or orange from the body/

#8. The mantle <of slugs, posterior or anterior>/

1. posterior, and covered by the small external shell/

2. anterior and large <enclosing the shell>/

Mantle of molluscs: the fleshy or membranous covering of that part of the body (usually anterior) which usually secretes the shell, and which encloses the ‘mantle cavity’. The latter functions as a lung, and connects with the exterior via a respiratory pore (pneumopore) in the mantle.

Illustrations of the principal structures referred to in routine descriptions of land and freshwater molluscs can be accessed via the ‘Mollusc morphology’ toolbar button.

#9. The mantle <patterning>/

1. exhibiting a large dorsal island, demarcated by a conspicuous groove/

2. with a fingerprint-like pattern of fine, concentric rings <folds>/

3. with neither a dorsal island demarcated by a groove, nor a fingerprint-like pattern of folds/

Mantle of molluscs: the fleshy or membranous covering of that part of the body (usually anterior) which usually secretes the shell, and which encloses the ‘mantle cavity’. The latter functions as a lung, and connects with the exterior via a respiratory pore (pneumopore) in the mantle.

Illustrations of the principal structures referred to in routine descriptions of land and freshwater molluscs can be accessed via the ‘Mollusc morphology’ toolbar button.

#10. The mantle <symmetry of the concentric rings>/

1. with the concentric rings centred on its mid-line <Limacidae>/

2. with the concentric rings centred to the right of the mid-line, above the respiratory pore <Agriolimacidae>/

#11. The animal <number of tentacles>/

1. with one pair of <retractable, but not invaginable> tentacles only/

2. with two pairs of <invaginable> tentacles/

#12. Eyes <presence>/

1. present <implicit>/

2. absent <the animal blind>/

#13. Eyes <position>/

1. at the tips of the <longer> posterior tentacles/

2. at the bases of the <only pair of> tentacles/

#14. The jaw <Zonitidae type>/

1. arcuate, with a vertical central keel ending in a median projection on the lower edge <Zonitidae and Limacidae only>/

2. not as in Zonitidae and Limacidae <implicit>/

#15. The jaw <entire or of distinct plates: Punctidae/Discidae>/

1. of a single piece <Discidae>/

2. of 19 separate plates <Punctidae>/

#16. The respiratory pore <pneumopore, in the slug’s mantle>/

1. towards the anterior right of the mantle/

2. towards the posterior right of the mantle/

Mantle of molluscs: the fleshy or membranous covering of that part of the body (usually anterior) which usually secretes the shell, and which encloses the ‘mantle cavity’. The latter functions as a lung, and connects with the exterior via a respiratory pore (pneumopore) in the mantle.

Illustrations of the principal structures referred to in routine descriptions of land and freshwater molluscs can be accessed via the ‘Mollusc morphology’ toolbar button.

#17. <Attachment to the substrate>/

1. attaching to stones, etc., via a byssus <a fibrous “beard”>/

2. without a byssus/

#18. The tail <of the slug, whether dorsally keeled>/

1. dorsally sharply keeled, the keel reaching to the mantle/

2. not dorsally keeled/

3. dorsally bluntly keeled, but the keel falling short of the mantle/

#19. The mantle <of the snail>/

1. partly covering the shell when the animal is in motion/

2. not extending over the shell <implicit>/

Mantle of molluscs: the fleshy or membranous covering of that part of the body (usually anterior) which usually secretes the shell, and which encloses the ‘mantle cavity’. The latter functions as a lung, and connects with the exterior via a respiratory pore (pneumopore) in the mantle.

Illustrations of the principal structures referred to in routine descriptions of land and freshwater molluscs can be accessed via the ‘Mollusc morphology’ toolbar button.

#20. The shell <univalve, presence of an operculum: see Notes>/

1. operculate <with a true operculum: see Notes>/

2. with a ‘clausilium’, which functions as an operculum when the animal retracts/

3. inoperculate <and without a clausilium>/

Operculum: a horny or calcareous plate, borne on the foot of Gastropoda-Prosobranchia, which seals the mouth of the shell when the animal retracts into it. It is not to be confused with the mucous plug (epiphragm) with which many land snails seal the shell when over-wintering or in order to withstand dry conditions.

Clausilium: a flexible, spoon-shaped plate, attached to the columella, which blocks the entrance when the animal withdraws. Characteristic of Clausiliidae.

Illustrations of the principal structures referred to in routine descriptions of land and freshwater molluscs can be accessed via the ‘Mollusc morphology’ toolbar button.

#21. The shell <when helically coiled, form>/

1. planispiral <with a flush or sunken spire>/

2. rising-spiral/

#22. The shell <spiral, number of whorls>/

whorled/

Illustrations of the principal structures referred to in routine descriptions of land and freshwater molluscs, and instructions for counting shell whorls, can be accessed via the ‘Mollusc morphology’ toolbar button.

#23. The <coiled> shell typically <sinistral or dextral>/

1. sinistral/

2. dextral/

'Sinistral' (left-handed) versus 'dextral' (right-handed) is readily ascertainable with reference to the direction of coiling of univalve shells. To assign one, hold it vertically with its spire on top, the umbilicus (if there is one) below, and the aperture towards you. The aperture of a dextral shell will then lie to the right of the median axis, that of a sinistral one to the left.

Many spiral-shelled species are liable to mutations that result in reversal of the direction of convolution, which involves the disposition of the internal organs as well as sinistrorsity or dextrorsity of the shell. Dextrorsity is much the commoner normal condition, but sinistrorsity characterizes Clausiliidae, Physidae and some Vertiginidae. The depressed spire of Planorbidae is not readily distinguishable from the umbilicus, but they are also correctly interpreted as sinistral with reference to the viscera.

Reversed individuals, which have sometimes attracted varietal status, are usually rare, but they seem to be commoner in some species than others; and in some cases (exemplified in Lymnaea peregra) the mutant sinistral condition is heritable, and viable to the extent that populations can persist for many years.

Illustrations of the principal structures referred to in routine descriptions of land and freshwater molluscs can be accessed via the ‘Mollusc morphology’ toolbar button.

#24. The animal <limpet, sinistral or dextral>/

1. sinistral, with the genital orifice and anus opening on its left side, although the vestigial spire of the shell is slightly twisted to the right/

2. dextral, with the genital orifice and anus on its right side, although the vestigial spire of the shell inclines to the left/

#25. The shell <maximum dimension when fully grown>/

mm in its maximum dimension <height or breadth>/

Illustrations of the principal structures referred to in routine descriptions of land and freshwater molluscs, and instructions for measuring heights and widths of shells, can be accessed via the ‘Mollusc morphology’ toolbar button.

#26. The shell <whether higher than wide>/

1. higher than wide/

2. about as high as wide/

3. wider than high/

Illustrations of the principal structures referred to in routine descriptions of land and freshwater molluscs, and instructions for measuring heights and widths of shells, can be accessed via the ‘Mollusc morphology’ toolbar button.

#27. The shell <height>/

mm high/

#28. The shell <width>/

mm wide/

#29. The <bivalve> shell <curvature>/

1. bent, with one margin quite sharply incurved and the other strongly curved outwardly/

2. oblong, with the margin opposite the hinge more or less straight or only slightly incurved/

3. not bent, the margin opposite the hinge outwardly curved/

#30. Height <of the shell, relative to its width> about/

x the width <shell height divided by width>/

Illustrations of the principal structures referred to in routine descriptions of land and freshwater molluscs, and instructions for measuring heights and widths of shells, can be accessed via the ‘Mollusc morphology’ toolbar button.

#31. The shell <raised-spiral type, relative dominance of the body (= last) whorl>/

1. with the body whorl predominating and the spire small and short/

2. high-spired and tapered gradually from the body whorl/

Illustrations of the principal structures referred to in routine descriptions of land and freshwater molluscs can be accessed via the ‘Mollusc morphology’ toolbar button.

#32. The height of the spire <relative to that of the shell> about/

x that of the shell <height of spire divided by height of shell>/

Illustrations of the principal structures referred to in routine descriptions of land and freshwater molluscs, and instructions for measuring heights widths of shell components, can be accessed via the ‘Mollusc morphology’ toolbar button.

#33. The spire <acute or obtuse>/

1. acute/

2. obtuse/

3. truncate/

Illustrations of the principal structures referred to in routine descriptions of land and freshwater molluscs can be accessed via the ‘Mollusc morphology’ toolbar button.

#34. The shell <symmetry>/

1. more or less symmetrical about its vertical axis <implicit>/

2. very asymmetric about the vertical axis, with the spire much displaced to one side/

Illustrations of the principal structures referred to in routine descriptions of land and freshwater molluscs, and of the terminology adopted here to describe shell shapes, can be accessed via the ‘Mollusc morphology’ toolbar button.

#35. The shell <wider than or about as wide as high, shape>/

1. discoid/

2. conical <pyramidal, implying more or less straight-sided>/

3. cochleate/

4. subglobose/

5. globose/

6. very asymmetic about its vertical axis, with a large body <= last> whorl and a small, displaced spire/

Illustrations of the principal structures referred to in routine descriptions of land and freshwater molluscs, and of the terminology adopted here to describe shell shapes, can be accessed via the ‘Mollusc morphology’ toolbar button.

#36. The shell <higher than wide: shape in outline, ignoring suturing of the spire>/

1. cylindrical/

2. fusiform/

3. ovoid-symmetric/

4. ovoid-asymmetric/

5. inverted-pyriform <pear-shaped>/

6. tear-shaped <cf. inverted-pyriform, but more gradually contracted into the spire>/

7. turretiform <elongated-conical, implying more or less straight-sided>/

Illustrations of the principal structures referred to in routine descriptions of land and freshwater molluscs, and of the terminology adopted here to describe shell shapes, can be accessed via the ‘Mollusc morphology’ toolbar button.

#37. The shell <whether deeply sutured>/

1. shallowly sutured/

2. deeply sutured/

Illustrations of the principal structures referred to in routine descriptions of land and freshwater molluscs can be accessed via the ‘Mollusc morphology’ toolbar button.

#38. The body whorl <of the shell, convexity>/

1. only slightly convex/

2. moderately convex/

3. very strongly convex/

#39. The whorls of the <elongated> spire <of the spired shell, convexity>/

1. feebly convex/

2. moderately convex/

3. very strongly convex/

#40. The whorls <of the shell, outline in section>/

1. neither shouldered nor keeled <implicit>/

2. shouldered/

3. keeled/

#41. The whorls <of the shell, whether spiny>/

1. exhibiting a median whorl of spines/

2. without spines <implicit>/

#42. The whorls <of the shell, whether transversely ridged>/

1. conspicuously and regularly transversely ridged <as distinct from striated> across the whorls/

2. not conspicuously transversely ridged <implicit>/

#43. The shell <elongate-spired, whether longitudinally striated>/

1. conspicuously and regularly striated across the whorls <i.e., along its length>/

2. not markedly striated across the whorls <implicit>/

#44. The aperture <of the shell, shape>/

1. round/

2. oval/

3. ovate/

4. lunate <crescentic>/

5. pear-shaped/

#45. The aperture <of the shell, teeth or calluses>/

1. bearing teeth/

2. bearing calluses/

3. with neither teeth nor calluses/

#46. The columella <smooth, folded or twisted>/

1. smooth/

2. folded/

3. twisted/

#47. The shell <presence of umbilicus>/

1. with an umbilicus/

2. without an umbilicus/

Umbilicus: a cavity at the base of those whorled, univalve shells in which the whorls do not form a solid axis.

Illustrations of the principal structures referred to in routine descriptions of land and freshwater molluscs can be accessed via the ‘Mollusc morphology’ toolbar button.

#48. The umbilicus <of the shell, size>/

1. small/

2. large and wide/

Illustrations of the principal structures referred to in routine descriptions of land and freshwater molluscs can be accessed via the ‘Mollusc morphology’ toolbar button.

#49. The shell <aperture lip, thickening>/

1. thick-lipped/

2. thin-lipped/

#50. The shell <consistency>/

1. thin and translucent/

2. <relatively thick and> opaque/

#51. The shell <ground colour>/

#52. The shells <colour details, Margaritiferidae/Unionidae>/

1. dull black, the interior pearly white or pinkish <Margaritiferidae>/

2. brown, bright or dark greenish or greenish brown, glossy <Unionidae>/

#53. The shell <patterned or plain>/

1. conspicuously colour-patterned/

2. plain/

Morphological comments

#54. <Comments on body colour>/

General biology, ecology

#55. <Terrestrial or aquatic>/

1. terrestrial/

2. freshwater aquatic/

3. semi-marine <in saltmarshes and brackish water, or around the high tide zone>/

This obviously useful character calls for judicious application for identification, with due regard to the habitat - “water snails” can be emergent in wet conditions, and many are equipped to withstand seasonal desiccation.

#56. <Respiratory organization>/

1. breathing via a single gill <ctenidium> attached within the mantle cavity/

2. breathing air directly via the lung-like mantle cavity <without gills: implicit>/

3. breathing via complex gills/

4. the mantle cavity obliterated, and a lobe of the mantle functioning as a gill/

#57. <Whether principally carnivorous>/

1. principally carnivorous/

2. not principally carnivorous <implicit>/

#58. Lifestyle <whether subterranean>/

1. subterranean/

2. not subterranean <implicit>/

#59. <Non-subterranean habitat, comments>/

#60. <Comments on diet>/

#61. <Sexuality>/

1. hermaphrodite/

2. the individuals either male or female (not hermaphrodite)/

#62. <Viviparous or oviparous>/

1. viviparous/

2. oviparous <implicit>/

#63. The stalk of the bursa copulatrix <presence of a diverticulum>/

1. with a diverticulum/

2. without a diverticulum/

#64. Courtship <whether involving love darts>/

1. involving exchanges of ‘love darts’ prior to mating/

2. not involving ‘love darts’ <implicit>/

Information on ‘love darts’, along with good illustrations, is available on the internet at: http://en.wikipedia.org/wiki/Love_dart#Occurrence_within_the_pulmonate_snails_and_slugs

Classification

#67. <Subclass of Gastropoda>/

1. Prosobranchia/

2. Pulmonata/

British representation

#68. <Genera in Britain, numbers of species, and common names>/

#69. <General comments>/

Miscellaneous

#70. Abbreviated taxon name:/

#71. <Illustrations>/


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’.

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