mussel shrimp, also called seed shrimp, or ostracod, any of a widely distributed group of crustaceans belonging to the subclass Ostracoda (class Crustacea) that resemble mussels in that the body is enclosed within a bivalved (two-valved) shell. Mussel shrimp differ from most other crustaceans in having a very short trunk that has lost its external segmentation, or divisions. The 6,650 living species include marine, freshwater, and terrestrial forms.
Some 10,000 extinct species are known, fossil ostracods being known from Cambrian Period (about 542 million to 488 million years ago) to recent times. The several extinct species are of particular ... (100 of 221 words)