Posted by Rob on February 08, 2004
I think that, as the previous contributors have identified, the cockle is a heart-shaped mollusc.
This leads to the combination of "Cockle" and "Heart". Indeed, in German, it is called the edible heart-mussel (essbare Herzmuschel).
We may also remember that a shell is a "coquille" in French - this is IMHO the origin of cockle since the similarity is striking. We English have the French to thank for a large part of our language ('education & intellectual words') and our Germanic forebears for much of the rest (everyday farmer life words).