The clade Aplysiomorpha, commonly known as sea hares (Aplysia species and related genera), are medium-sized to very large Opisthobranchia with a soft internal shell made of protein. These are marine gastropod molluscs in the superfamilies Aplysioidea and Akeroidea.
The common name “sea hare” is direct translation from Latin lepus marinus, as the animal was called so already in Roman times. The name derives from their rounded shape and from the two long rhinophores that project upwards from their heads and that somewhat resemble the ears of a hare. source