A subclass in the class Gastropoda containing about 4000 living species, arranged in nine orders, including the herbivorous Aplysiomorpha (sea hares) and Sacoglossa and the carnivorous Thecosomata (sea butterflies); Nudibranchia (sea slugs); Cephalaspidea (or Bullomorpha); Gymnosomata; Pleurobranchomorpha (or Notaspidea); Acochlidiacea; and Runciniodea. Primitive members of many of the orders show adaptations for burrowing beneath sand or mud; more advanced members are always active surface-living or pelagic forms. This trend is accompanied by a decrease in the importance of the shell and operculum for passive defense. These are replaced by more dynamic chemical (some species secrete decinormal sulfuric acid through the skin if annoyed), physical (daggerlike calcareous epidermal spicules), or biological (redirected nematocysts derived from coelenterate prey) defensive mechanisms.