Snakehead, crab, dolphin, shell and lobster choose shells because they are different from the other four. According to relevant data, it can be clearly seen from these four words that shells are different from the other four and can be divided into two categories. Shells belong to shellfish, while snakehead, crab and lobster belong to fish. The reason for choosing shells is different from the other three, but they all live in the sea. They are different species. There are many species in the ocean, including shellfish, fish and algae.