Mantle with two posterior appendages; ctenidium large and capable of protrusion from pallial cavity.
Thus in the Phylactolaemata the contraction of the muscular body-wall exerts a pressure on the fluid of the body-cavity and is the cause of the protrusion of the polypide.
In the Gymnolaemata protrusion is effected by the contraction of the parietal muscles, which pass freely across the body-cavity from one part of the body-wall to another.
The mechanism of protrusion in the Cyclostomata is a subject which requires further examination.
2), or membranaceous and polished, hairy or smooth, oval or round or bean-shaped, or of some less simple pattern; the valves may fit neatly, or one overlap the other, their hinge may have teeth or be edentulous, and their front part may be excavated for the protrusion of the antennae or have no such " rostral sinus."