Sea Slugs with shells?

Dear Amanda,
Have a look at the page on how snails became slugs. It will give you links to other relevant pages. The basic point is that 'slugs' have evolved many times from snails through the gradual downgrading in importance of the external shell, until finally the shell is completely lost in the adult, although often still present in the larvae. Of all the sea slug groups, it is only in the nudibranchs that the shell is absent in all adults. In other groups, such as the cephalaspidean 'Bubble-shells', such as Pupa coccinata, an external shell is often present, while in the sacoglossans, we have a continuum from animals with an external shell, such as Volvatella ayakii to 'nudibranch'-like species, such as Stiliger smaragdinus, in which there is no trace of a shell.
Best wishes,
Bill Rudman