Let's Collect Rocks and Shells - Let's Collect Rocks and Shells is a book related to Nature. The book discussed about Rocks and Shells. First, a seashell is one of the 100, 000 species of backboneless animals belonging to the zoological group known as the Mollusca. Mollusks include not only the familiar clams, scallops and snails, but also the squids, octopus and Chambered Nautilus. Other "shells" found in the ocean include those of crabs, lobsters, barnacles and sea urchins.