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Sea slug is a common name used for several different groups of saltwater snails that either lack a shell or have only an internal shell. It is a paraphyletic name used for various lineages of marine gastropod molluscs that are either not conchiferous (shell-bearing) or appear not to be.
The phrase "sea slug" is often applied to nudibranchs (many members of which are colorful and are a noticeable part of the underwater fauna), sea hares, the sacoglossans, various families of bubble snails (Cephalaspidea), the sorbeoconch family Pterotracheoidea, the pulmonate (air-breathing) sea slug family Onchidiidae, and others.
In his book 'Evenings at the Microscope' P H Gosse says the following. “ If you ask what can be the use of ears to a class of animals which are invariably dumb, I answer that though this is true with respect to the great majority, yet it may be only that our senses are too dull to perceive the delicate sounds which they utter, and which may be sufficiently audible to their more sensitive organs; and besides, some Mollusca can certainly emit sounds audible by us. Two very elegant species of Sea-slug, viz., Eolis punctata, and Tritonia arborescens, certainly produce audible sounds. Professor Grant, who first observed the interesting fact in some specimens of the latter which he was keeping in an aquarium, says of the sounds, that " they resemble very much the clink of a steel wire on the side of the jar, one stroke only been given at a time, and repeated at intervals of a minute or two; when placed in a large basin of water the sound is much obscured, and is like that of a watch, one stroke being repeated, as before, at intervals. The sound is longest and oftenest repeated when the Tritonia) are lively and moving about, and is not heard when they are cold and without any motion ; in the dark I have not observed any light emitted at the time of the stroke ; no globule of air escapes to the surface of the water, nor is any ripple produced on the surface at the instant of the stroke ; the sound, when in a glass vessel, is mellow and distinct." The Professor has kept these Tritonire alive in his room for a month, and during the whole period of their confinement they have continued to produce the sounds with very little diminution of their original intensity. In a small apartment they are audible at the distance of twelve feet. " The sounds obviously proceed from the mouth of the animal ; and at the instant of the stroke, we observe the lips suddenly separate, as if to allow the the water to rush into a small vacuum formed within. As these animals are hermaphrodites, requiring mutual impregnation, the sounds may possibly be a means of communication between them ; or, if they are of an electric nature, they may be the means of defending from foreign enemies one of the most delicate, defenceless, and beautiful Gasteropods that inhabit the deep." 
- Fresh Science http://freshscience.org.au/2007/how-sea-slugs-fall-in-love
- Frontiers in Zoology http://www.frontiersinzoology.com/content/4/1/17
- Sea Slug Forum http://www.seaslugforum.net/