Subclass Opisthobranchia, Sea Slugs, Largely are missing or have reduced shells. Have other (chemical, biological defenses), camouflage. Comprise five living Orders: Anaspidea (Sea Hares), Cephalaspidea (Bubble Shells and Headshield Slugs), Notospidea (Side-Gilled Slugs), Nudibranchia (Nudibranchs) and Sacoglossa (Sap-Sucking Slugs). Opisthobranch means "gills behind" where most of these gastropods have their gills and anus situated (as opposed to the Prosobranch snails with their anus and gills up front). Opisthobranchs are hermaphroditic, both functional females and males in one body, though cross-fertilization is the rule. Laid eggs in clusters or bands hatch out to pelagic trochophore larvae which metamorphose into veligers, finally settling down as miniatures of their parents. There are some 2,000 species worldwide, and about 150 of these can be found in Hawaiian waters. 

Order Anaspidea, Sea Hares: Named for their rabbit ear-like projections called rhinophores (used for taste, current detection), and two anterior-projecting oral tentacles. Some can swim in the water column. Herbivorous, eating Red, Green, Brown Algae and Seagrasses. Can be dangerous in captivity if disturbed to the point of releasing fish-repelling purple "ink"...

Family Aplysiidae: In old Hawai'i sea hares were called Kualakai and some were cooked in an imu wrapped in ti leaves... and consumed.

Order Cephalaspidea, the Headshield Slugs: Worldwide, often have shells (often reduced, internal), often with operculums. Burrowing marine forms mainly that eat bristleworms, other worms, foraminiferans, bivalves, snails. Named for wedge-shaped heads that many use to plow through the sand, just below its surface.