After a good thunderstorm
For video: http://youtu.be/pK7ra4S36aE
Aproximately 70mm (2 and 3/4 in) long. Spotted at midnight. Known in Japan as Kougaibiru.
Here are two examples of 1.5" Kaburakia Excelsa or Giant Flatworms. In one photo, the worm is lying flat inside a clamshell, and in the other it's to the left of the chiton and the Plumose Anemone.
Tentative ID from John Hoover is family Euryleptidae.
The flatworms, known in scientific literature as Platyhelminthes or Plathelminthes (from the Greek πλατύ, platy, meaning "flat" and ἕλμινς (root: ἑλμινθ-), helminth-, meaning worm) are a phylum of relatively simple bilaterian, unsegmented, soft-bodied invertebrate animals. Unlike other bilaterians, they have no body cavity, and no specialized circulatory and respiratory organs, which restricts them to having flattened shapes that allow oxygen and nutrients to pass through their bodies by diffusion. The digestive cavity has only one opening...