Young Brunette Girl Wearing A Seashell Necklace

Some echinoderms such as sea urchins, including heart urchins and sand dollars, have a hard "test" or shell. After the animal dies, the flesh rots out and the spines fall off, and then fairly often the empty test washes up whole onto a beach, where it can be found by a beachcomber. These tests are fragile and easily broken into pieces.
• Brachiopods
The brachiopods, or lamp shells, superficially resemble clams, but the phylum is completely unrelated to mollusks. Most lines of brachiopods ended during the Permian-Triassic extinction event, and their ecological niche was filled by bivalves. A few of the remaining species of brachiopods occur in the low intertidal zone and thus can be found live by beachcombers.