The ctenidium is atrophied, and the edge of the mantle-skirt is fused to the dorsal integument by concrescence, except at one point which forms the aperture of the mantle-chamber, thus converted into a nearly closed sac. Air is admitted to this sac for respiratory and hydrostatic purposes, and it thus becomes a lung.
Section across the axis of a ctenidium with a pair of plates - flattened and shortened filaments - attached.
Foot without parapodia; no pallial cavity, but always a single ctenidium situated on the right side between mantle and foot.
Arca and Pectunculus) the lateral processes which are set on the axis of the ctenidium are not lamellae, but are slightly flattened, very long tubes or hollow filaments.
Shell absent in the adult; no ctenidium or osphradium.
The ctenidium (branchial plume).
I, Ctenidium (gill-plume).
I,j, k,g Are placed on or near the membrane which attaches the axis of the ctenidium to the side of the body.
This organ has, without reason, been supposed to represent the second ctenidium of the typical mollusc, which it cannot do on account of its position.
Where the modification is carried to its extreme degree, not only the shell but the pallial cavity, ctenidium and visceral hump disappear, and the body acquires a simple elongated form and a secondary external symmetry, as in Pterotrachaea and in Doris, Eolis, and other Nudibranchia.