Ostracoda are the most successful crustacean groups with approximately 8000 living species.  They are generally small in size ranging in length from 0.1 to 32 mm, which is smaller than a poppy seed to the size of a meatball.  Gigantocypris is a planktonic ostracod and is the largest at a size of 32mm. It is the only ostracod found swimming in the open water.  All  other ostracods are usually found burrowing into the sediments at the bottoms of oceans and lakes. few species like Mesocypris are also found crawling around in moist terrestrial habitats like mosses 

Their bodies consist of a tiny bit more than a head, and have the typical five pairs of appendages on their head. However, they only have 1-3 pairs of legs on the rest of the body. They have a bivalved carapace, or a exoskeleton that can cause them to be mistaken them for small clams or mussels. This is how they got their common name of "mussel shrimp".  Ostracods also shed their carapace with each molt as they grow. (Ben Waggoner)

On a fun Note:
The ostracods have the most complete fossil record of any of the crustaceans.