Mussel Shrimp: Ostracoda

The bodies of mussel shrimp are completely surrounded by a folded, shieldlike carapace (CARE-eh-pes) that resembles an upside down taco. Depending on the species, the carapace may be smooth, bumpy, pitted, bristly, or spiked. The front of the carapace may or may not have a beaklike projection. The two halves of the carapace are not the same size and one side fits snugly inside the other. They have fewer body segments and appendages than other crustaceans. Most species are 0.08 inches (2 millimeters) or less, but some "giant" species measure 1.26 inches (32 millimeters). Ostracods resemble clam shrimp, but their shells do not have growth rings.

The compound eyes are not set on stalks, if they have eyes at all. Each compound eye has multiple lenses. There are two pairs of antennae and three pairs of mouthparts. The first pair of antennae, the antennules (an-TEN-yuls), is uniramous (YU-neh-RAY-mus), or not branched. The second pair of antennae is branched, or biramous (BY-ray-mus). Adults use their feathery antennae, and sometimes the antennules, for swimming. The thorax and abdomen usually do not have segments and look very similar to each other. There are one to three pairs of biramous limbs. Depending on the species, these limbs are used for walking or cleaning. The tip of the abdomen has a pair of slender or platelike appendages.