Sea slug

Sea slug is an informal name for a group of gastropods which look similar, but are not monophyletic. They are polyphyletic. Their similar shape and life styles is an example of convergent evolution. The group includes:
It would seem that their similar growth pattern, with the loss of torsion, bilateral symmetry, reduction of the Gastropod shell, and somewhat similar life styles, evolved a number of times in the Gastropods.