Nudibranch Evolution

Dear Valerie,
I can give you a small answer to a very big question. Firstly I can't really see the difference in the two alternatives you ask about - to diverge they would  need first to be together.

The important first thing to realise is that a 'slug' is a body shape or 'grade' of organisation not an evolutionary group. For example sharks, bony fish, dolphins are all 'fish' in the sense that they all have a fish grade of organisation, but they are not closely related to each other in evolutionary terms. The slug shape has independently evolved many times in the gastropod molluscs from shelled snails. Have a look at the 'What is a slug ?' Fact Sheet. We still have much to learn about the evolution of gastropods, but the general consensus at present is that the Opisthobranchs and Pulmonates - that is the animals that make up the sea slugs and the land snails (& slugs) - have a common ancestry, being descended from a special group of marine snails with peculiarities in their nervous system, larval shell, radula and mantle cavity, which set them apart from all other marine snails.

So what we can say is that sea slugs and land slugs have a common ancestor, but the  huge diversity we see today is the result of many evolutionary events which include the independent development of the slug form many times.

Best wishes,
Bill Rudman