Mystacocarids: Mystacocarida

Mystacocarids (my-stah-koh-KAR-ids) are small, wormlike crustaceans that reach up to 0.039 inches (1 millimeter) in length. Nearly one-third of their entire body length is made up of a head that is not covered with a shieldlike carapace (CARE-eh-pes). The eyes are simple and are not set on stalks. Each simple eye has only one lens. The head is sharply narrowed between the pairs of antennae. The first pair of antennae, or antennules (an-TEN-yuhls), is uniramous (YU-neh-RAY-mus), or not branched. The antennules are about half as long as the entire body. The second pair of antennae is branched, or biramous (BY-ray-mus). The jaws, or mandibles, are also biramous, while the second set of mouthparts, or maxillae (mack-SIH-lee), are uniramous. There are long, hairlike structures on the inside margin of the maxillae. The thorax has five segments. The first thoracic segment is not tightly joined, or fused, with the head. It has a pair of biramous limbs called maxillipeds (mack-SIH-leh-pehds). Maxillipeds work together with the mouthparts. Each of the four remaining thoracic segments has a pair of leglike limbs called pereopods (PAIR-ee-oh-pawds). The uniramous pereopods are sometimes used for grasping. The five abdominal segments do not have any appendages underneath. The tip of the abdomen ends in a large taillike segment, or telson. The telson is fused to the last abdominal segment. A well developed bladelike structure is found on either side of the telson.