Sea slug

Sea slug
Nembrotha kubaryana feeding on tunicates
Scientific classification
Kingdom:	Animalia
Phylum:	Mollusca
Class:	Gastropoda
(unranked):	clade Heterobranchia

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:

• Heterobranchia
  • Nudibranchs: a clade with bilateral symmetry
  • Sacoglossa, a clade of Heterobranch sea slugs and snails which go in for kleptoplasty ('stealing' plastids). They eat algae and keep the chloroplasts for their own use.
  • Opisthobranchs: a long-used term now just an informal label.
    • Sea angels, or cliones, are opisthobranch gastropod molluscs of the clade Gymnosomata.
    • Sea hares are medium to large size marine gastropod molluscs. They belong to two families of the Opisthobranchia.
    • Cephalaspidea: a sub-order of Opisthobranch gastropod molluscs. They usually have a reduced internal shell.
• Onchidiidae: a family of air-breathing sea slugs.
  • Sea butterflies: a group of pelagic sea snails sometimes called pteropods. They belong to the clade Thecosomata.

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.