| Sea urchins, like bivalve molluscs, are cosmopolitan in their distribution. |
| A bivalve is characterized by possessing two shells secreted by a mantle that extends in a sheet on either side of the body. |
| Now explain how a univalve differs from a bivalve, in more ways than just the number of shells. |
| The beach sands are dominated by shells of bivalve mollusks, mainly venerids, gastropods, and echinoderms. |
| Ours, however, is closed along the plane of bilateral symmetry, resembling the two closed valves of a bivalve rather than being flattened. |
| Mactridae, also known as surf or trough shells or duck clams, is a family of marine bivalve clams of the order Veneroida. |
