They are considered very primitive opisthobranchs. The shell of these micromollusks is gradually reduced in consecutive order from Acteonidae, to Bullidae, to Hydatinidae. They all have first an external penis; afterwards they develop the female part (a process called protrandry). They are distinguished from other opisthobranchs, by having a headshield, lack of tentacles and broad parapodia (mantle flaps), which fold along the side of the shell as the mollusk crawls on a sandy bottom. They are carnivores, preying on polychaete worms.
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