I found it crawling right out of the water on a rock in the intertidal zone. I was so worried about it that I popped it into the pool. This was not the right thing to do because it immediately crawled out again. Dear Valda, This is a Pulmonate Slug . It hides in crevices at high tide emerging at low tide to graze over rocks.
Studies show that it follows a mucous trail home after each feeding 'expedition'.
I remember, in northwestern India, where I saw these animals alive for the first time, wondering what these large warty dorids were doing so high on the shore. As you can see in the bottom right photo, they have remarkable gill-like branching papillae over the posterior half of their backs, and it is only on picking them up that their pulmonate anatomy becomes obvious. J. exp. mar. Comment on Giant Onchidium from South Africa by Valda Fraser. Dear Erwin, Yes they are a species of the pulmonate air-breathing family Onchidiidae.