Telltale Shell Parts: The Pallial Sinus
The pallial sinus is an etched “bent line” present on the inside of some bivalve shells, at their posterior end. The sinus is a continuation of the pallial line, a scar that indicates the limit of the bivalve body that is attached internally to its shell.
The pallial sinus corresponds to a "pouch" in the animal body that houses the siphons when they are retracted. The siphons are fleshy tubes that can be expanded for the intake and outtake of water for respiration and feeding. Usually, bivalves that burrow deep in the sand or mud have long siphons; the longer the siphons, the deeper the pallial sinus!