Sluglike marine mollusks with no trace of a shell, ranging in size from 0.4-3.0 in (10-70 mm) long. The dorsal surface is covered by a tough, leathery mantle with a bumpy, rocky-looking surface. The mantle contains an air-breathing lung for use out of water, and has a pneu-mostome in the posterior region, near the anus, which is also posteriorly placed. When underwater, the animal relies for respiration on its skin and on gill-like protrusions on its dorsal surface. A most interesting and little-understood feature is an array of light-sensitive papillae on the dorsal surface.
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