I found this rather interesting from Wikipedia regarding these slugs ...
'The sacoglossans can utilise the chloroplasts of the algae on which they feed, which they keep alive for hours to months after their ingestion. They maintain the cells and metabolise the photosynthetic products;[11] this process is termed kleptoplasty, and the sacoglossans are the only animals to employ it; some ciliates and foramanifera (protists) also employ the strategy.[10] Sacoglossans have been known to survive for months living solely on the photosynthetic products of their acquired plastids.[10] This process is somewhat mystifying, as the upkeep of chloroplasts usually requires interaction with genes encoded in the plant cell nucleus. This almost seems to suggest that the genes have been laterally transferred from algae to the animals.'