Slug (British Band) Albums

Slug, or land slug, is a common name for any apparently shell-less terrestrial gastropod mollusc. The word ''slug'' is also often used as part of the common name of any gastropod mollusc that has no shell, a very reduced shell, or only a small internal shell, particularly sea slugs and semi-slugs (this is in contrast to the common name '' snail'', which applies to gastropods that have a coiled shell large enough that they can fully retract their soft parts into it).

Various taxonomic families of land slugs form part of several quite different evolutionary lineages, which also include snails. Thus, the various families of slugs are not closely related, despite the superficial similarity in overall body form. The shell-less condition has arisen many times independently as an example of convergent evolution, and thus the category "slug" is polyphyletic.

Taxonomy

Of the six orders of Pulmonata, two – the Onchidiacea and Soleolifera – solely comprise slugs. A third group, the Sigmurethra, contains various clades of snails, semi-slugs (i.e. snails whose shells are too small for them to retract fully into), and slugs. The taxonomy of this group is in the process of being revised in light of DNA sequencing. It appears that pulmonates are paraphyletic and basal to the opisthobranchs, which are a terminal branch of the tree. The family Ellobiidae are also polyphyletic.

*Subinfraorder Orthurethra
**Superfamily Achatinelloidea Gulick, 1873
**Superfamily Cochlicopoidea Pilsbry, 1900
**Superfamily Partuloidea Pilsbry, 1900
**Superfamily Pupilloidea Turton, 1831
*Subinfraorder Sigmurethra
**Superfamily Acavoidea Pilsbry, 1895
**Superfamily Achatinoidea Swainson, 1840
**Superfamily Aillyoidea Baker, 1960
**Superfamily Arionoidea J.E. Gray in Turnton, 1840
**Superfamily Athoracophoroidea
***Family Athoracophoridae
**Superfamily Orthalicoidea
***Subfamily Bulimulinae
**Superfamily Camaenoidea Pilsbry, 1895
**Superfamily Clausilioidea Mörch, 1864
**Superfamily Dyakioidea Gude & Woodward, 1921
**Superfamily Gastrodontoidea Tryon, 1866
**Superfamily Helicoidea Rafinesque, 1815
**Superfamily Helixarionoidea Bourguignat, 1877
**Superfamily Limacoidea Rafinesque, 1815
**Superfamily Oleacinoidea H. & A. Adams, 1855
**Superfamily Orthalicoidea Albers-Martens, 1860
**Superfamily Plectopylidoidea Moellendorf, 1900
**Superfamily Polygyroidea Pilsbry, 1894
**Superfamily Punctoidea Morse, 1864
**Superfamily Rhytidoidea Pilsbry, 1893
***Family Rhytididae
**Superfamily Sagdidoidera Pilsbry, 1895
**Superfamily Staffordioidea Thiele, 1931
**Superfamily Streptaxoidea J.E. Gray, 1806
**Superfamily Strophocheiloidea Thiele, 1926
**Superfamily Parmacelloidea
**Superfamily Zonitoidea Mörch, 1864
**Superfamily Quijotoidea Jesús Ortea and Juan José Bacallado, 2016
***Family Quijotidae

Description

The external anatomy of a slug includes the following:

* Tentacles: Like other pulmonate land gastropods, the majority of land slugs have two pairs of 'feelers' or ''tentacles'' on their head. The upper pair is light-sensing and has eyespots at the ends, while the lower pair provides the sense of smell. Both pairs are retractable in stylommatophoran slugs, but contractile in veronicellid slugs.
* Mantle: On top of the slug, behind the head, is the saddle-shaped ''mantle''. In stylommatophoran slugs, on the right-hand side of the mantle is a respiratory opening, the pneumostome, which is easier to see when open; also on the right side at the front are the genital opening and anus. Veronicellid slugs have a mantle covering the whole dorsal part of the body, they have no respiratory opening, and the anus opens posteriorly.
* Tail: The part of a slug behind the mantle is called the 'tail'.
* Keel: Some species of slugs, for example ''Tandonia budapestensis'', have a prominent ridge running over their back along the middle of the tail (sometimes along the whole tail, sometimes only the posterior part).
* Foot: The bottom side of a slug, which is flat, is called the 'foot'. Like almost all gastropods, a slug moves by rhythmic waves of muscular contraction on the underside of its foot. It simultaneously secretes a layer of mucus that it travels on, which helps prevent damage to the foot tissues. Around the edge of the foot in some slugs is a structure called the 'foot fringe'.
* Vestigial shell: Most slugs retain a remnant of their shell, which is usually internalized. This organ generally serves as storage for calcium salts, often in conjunction with the digestive glands. An internal shell is present in the Limacidae and Parmacellidae. Adult Philomycidae, Onchidiidae and Veronicellidae lack shells.

Physiology

Slugs' bodies are made up mostly of water and, without a full-sized shell, their soft tissues are prone to desiccation. They must generate protective mucus to survive. Many species are most active just after a rain because of the moist ground or during nighttime. In drier conditions, they hide in damp places such as under tree bark, fallen logs, rocks and manmade structures, such as planters, to help retain body moisture. Like all other gastropods, they undergo torsion (a 180° twisting of the internal organs) during development. Internally, slug anatomy clearly shows the effects of this rotation—but externally, the bodies of slugs appear more or less symmetrical, except the pneumostome, which is on one side of the animal, normally the right-hand side.

Slugs produce two types of mucus: one is thin and watery, and the other thick and sticky. Both kinds are hygroscopic. The thin mucus spreads from the foot's centre to its edges, whereas the thick mucus spreads from front to back. Slugs also produce thick mucus that coats the whole body of the animal. The mucus secreted by the foot contains fibres that help prevent the slug from slipping down vertical surfaces.

The "slime trail" a slug leaves behind has some secondary effects: other slugs coming across a slime trail can recognise the slime trail as produced by one of the same species, which is useful in finding a mate. Following a slime trail is also part of the hunting behaviour of some carnivorous slugs. Body mucus provides some protection against predators, as it can make the slug hard to pick up and hold by a bird's beak, for example, or the mucus itself can be distasteful. Some slugs can also produce very sticky mucus which can incapacitate predators and can trap them within the secretion. Some species of slug, such as ''Limax maximus'', secrete slime cords to suspend a pair during copulation.

Reproduction

Slugs are hermaphrodites, having both female and male reproductive organs. Once a slug has located a mate, they encircle each other and sperm is exchanged through their protruded genitalia. A few days later, the slugs lay approximately thirty eggs in a hole in the ground, or beneath the cover of an object such as a fallen log.

Apophallation has been reported only in some species of banana slug (''Ariolimax'') and one species of ''Deroceras''. In the banana slugs, the penis sometimes becomes trapped inside the body of the partner. Apophallation allows the slugs to separate themselves by one or both of the slugs chewing off the other's or its own penis. Once the penis has been discarded, banana slugs are still able to mate using only the female parts of the reproductive system.

In a temperate climate, slugs usually live one year outdoors. In greenhouses, many adult slugs may live for more than one year.

Ecology

Slugs play an important role in the ecosystem by eating decaying plant material and fungi. Most carnivorous slugs on occasion also eat dead specimens of their own kind.

Feeding habits

Most species of slugs are generalists, feeding on a broad spectrum of organic materials, including leaves from living plants, lichen