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Why are the two valves of a lamp shell unequal in size?

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lamp shells Lamp shells on the sea floor off the coast of Massachusetts.

The two valves of a lamp shell are unequal in size because they correspond to the distinct dorsal (top) and ventral (bottom) sides of the animal's body, rather than being mirror images of each other like the valves of a clam.1

While the valves differ in size, the animal itself exhibits bilateral symmetry, meaning the left and right sides of the shell are mirror images of one another. It is important to note that not all brachiopods possess unequal valves; some groups, such as those in the order Lingulida, are biconvex, meaning both valves are similar in shape and size.12