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The heart is usually posterior to the branchia (proso-branchiate).
In these there are neither branchia nor osphradium, and the pallial chamber which retains its large open ing serves as a lung.
Head very large, not marked off from the body; neither branchia nor suckers; fins situated near the middle of the body.
External to the branchia are seen ten club-like processes of the dorsal wall, these are the " cerata " which are characteristically developed in another sub-order of Opisthobranchs.
Visceral mass and shell conical; head flattened; pallial cavity aquatic, but without a branchia; genital apertures separated.