Cockles of my heart

Anybody know where this phrase originated and what it means?
"warms the cockles of my heart"

Meaning: Causes a pleasant feeling of a sentimental kind, is comforting, is reassuring.
Origin: Not clear. The shell of the cockle, a mussel, is somewhat heart-shaped, but that may not have produced this phrase.
Entry from Webster's Second Unabridged, 1934: "Cockles of the heart: A phrase (in which 'cockles' is of uncertain meaning) denoting the depths of the heart; as, to 'delight, rejoice, cheer, warm,' etc., 'the cockles of one's heart.'"

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