Plakobranchoidea
The majority of sacoglossans are shell-less, consequently the placobranchoidea are commonly described using the vernacular term "sea slugs", which can lead to their confusion with the only very distantly related nudibranchs. However, the plakobranchoid Elysia (and undoubtedly others) do develop a shell before hatching from its egg. Indeed, at least the Elysiidae, Limapontiidae and Hermaeidae all bear larval shells, which are spiral, and possess between three-quarters of a whorl and one complete whorl.
The plakobranchoids have a more diverse feeding range than the Oxynoacea, feeding on a wider range of green (and sometimes red) algae and even, in three cases, being carnivorous.
Read more about this topic: Sacoglossa