What is it?

I was reading over a paper by John Peyssonel from the Royal Society’s Philosophical Transactions (volume 50, 1757-1758, pages 585-589) titled ‘Observations on the limax non cochleata purpur ferens, the naked snail producing purple.’ It is about some sea creature and after reading it I have no clue what it is.

From the text (which is longest run on sentence I’ve ever seen):

Among the fish we meet with in the seas of the Antilles of America, we find, that this I am going to describe will appear precious, from the beautiful purple colour it produces, in the same manner, that the cuttle-fish produces its ink, if a means could be found to produce the liquor in a sufficient quantity to render it an article of commerce

The author goes on to describe this ‘fish’ as soft, viscous, without shells, scales or bones. It has no feet or fins. It acts like a slug when touched, in that it wreaths up as round as it can. In fact, they are so similar to snails and slugs the author calls them ‘naked snails.’ Their bodies are greenish in colour with black circular spots. They have two horns or antennae which might serve as eyes. Under a tough plate at the back of the body, it keeps a sack of purple juice. The purple juice can be deployed in defense just like a cuttle-fish uses it’s ink.

Is this a description of a nudibranch? Or maybe a sea hare?