In order to help you i’ll need to see the diagram but Nudibranchs are often casually called sea slugs, as they are a family of opistobranchs (sea slugs), sea slug is a common name standing for many types of these animals. they are sea snails (marine gastropod mollusks) that over evolutionary time have either completely lost their shells, or have seemingly lost their shells due to having a greatly reduced or internal shell.
so answering you question i would say they are related to snails:)
