Posterior branchia triradiate.
The heart is usually posterior to the branchia (proso-branchiate).
The sub-class is now divided into two orders: the Aspidobranchia in which the branchia or ctenidium is bipectinate and attached only at its base, and the Pectinibranchia in which the ctenidium is monopectinate and attached to the mantle throughout its length.
In these there are neither branchia nor osphradium, and the pallial chamber which retains its large open ing serves as a lung.
Such adaptations are the transparency and colourlessness of the tissues, and the modifications of the foot, which still shows in Atlanta the form common in Pectin:branchia (compare fig.
Head very large, not marked off from the body; neither branchia nor suckers; fins situated near the middle of the body.
No branchia FIG.
No branchia; two long and branched buccal appendages.
Visceral mass and shell conical; head flattened; pallial cavity aquatic, but without a branchia; genital apertures separated.
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.