Lamellariids, or as they now should be called, velutinids,(formerly Family Lamellariidae), are caenogastropod snails, closely related to the Triviidae. In most velutinids the shell is open and flattened, much like a small abalone shell (Haliotidae), and at least partially enveloped by the fleshy mantle. This gives them a fleshy shell-less appearance which often causes them to be mistaken for sea slugs. In most cases when they are crawling a pair of head tentacles will emerge from beneath the body, instantly showing that these are not opisthobranch sea slugs. Another character is the anterior siphon above the head which opens into the mantle cavity. Source: http://www.seaslugforum.net/lamellar.htm...
Coral reefs, tropical Indo-West Pacific
First time to spot one and actually didn't know what it was then. Thanks to Kathrin Hachenberg for the ID in fb. About the size of a clenched fist. Depth of about 4m. Night Dive, Batangas, Philippines