Oxynoacea
Around 20% of sacoglossan species bear a shell. The Oxynoacea contains three shelled families, and all feed solely on algae of the genus Caulerpa. None of these organisms benefit from the photosynthesis of the ingested chloroplasts, but there is some suggestion that the chloroplasts may have been retained in order to perform a camouflaging function. The shells of the Volvatellidae and Oxynoidae somewhat resemble those of the Cephalaspid bubble snails. The Juliidae are extraordinary in that they are shelled, bivalved, gastropods. They have a shell in two pieces which resemble the valves of a minute clam. Living members of this family has been known since 1959, and had previously only been known to science as fossils (which had been interpreted as bivalves).
Read more about this topic: Sacoglossa