Azure A's List: Mollusca

• The various taxonomic families of slugs form part of several quite different evolutionary lineages which also include snails. Thus, for example, the various families of land slugs are not very closely related to one another, despite a superficial similarity in the overall body form. The shell-less condition has arisen many times independently during the evolutionary past, and thus the category "slug" is emphatically a polyphyletic one.
• Slugs, like all other gastropods, 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 land slugs appear to be more or less symmetrical, except for the positioning of the pneumostome, which is on one side of the animal, normally the right hand side.
• 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, and can be regrown if lost.
• On top of the slug, behind the head, is the saddle-shaped mantle, and under this are the genital opening and anus. On one side (almost always the right hand side) of the mantle is a respiratory opening, which is easy to see when open, but difficult to see when closed. This opening is known as the pneumostome. Within the tissue of the mantle in some species is a very small, rather flat shell, or in some other cases a collection of calcareous granules.
• Like most other gastropods, a slug moves by rhythmic waves of muscular contraction on the underside of its foot. It simultaneously secretes a layer of mucus on which it travels, which helps prevent damage to the foot tissues.^[3] Around the edge of the foot is the 'foot fringe' or 'skirt'.

• 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.

• Slugs are hermaphrodites, having both female and male reproductive organs.^[10] 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 around 30 eggs in a hole in the ground, or beneath the cover of an object such as a fallen log.

   

  Apophallation is a commonly seen practice among many slugs. In apophallating species, the penis curls like a corkscrew and during mating, it often becomes entangled in the mate's genitalia. Apophallation allows the slugs to separate themselves by one or both of the slugs chewing off the other's penis. Once its penis has been removed, the slug is still able to mate using only the female parts of its reproductive system.

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