Species of Doris?

Bequia, St Vincent and the Grenadines. We often find these tiny critters on the underside of dead sand dollars - are they a Doris?

They look more like flatworms.

Maybe the netted flatworm? I've found a large one of those on sand dollars before too.

Not up to speed on flatworms and can't find much on Caribbean species except here - http://www.rzuser.uni-heidelberg.de/~bu6/car_gall01.html

Could you post a blow-up of just the worm?