Best wishes, Bill Rudman. The animal is about 2 cm long and has a dark internal shell. Sorry about the quality of the shots. Thanks in advance. Dear David, This is not a slug with an internal shell but a snail with an internal shell.
Some snails have flaps of skin which partially or fully enclose the shell.
Your question has made me prepare pages on the cowries and false cowries so have a look at the three relevant pages for some background information. I first thought your animal was a velutinid, but your mention of a dark visible shell makes me think you have a cowry. Unfortunately I can't make out enough detail to be sure. Best wishes, Bill Rudman. Comment on Sea slug with internal shell by David Freemantle.