Leptostracans: Phyllocarida

Most leptostracans are 0.19 to 0.59 inches (5 to 15 millimeters) in length, but the largest species measure up to 1.96 inches (50 millimeters). Their transparent bodies are covered by a loose whitish shield, or carapace, that is folded over their backs. The carapace (CARE-eh-pes) is flattened from side to side and covers the thorax or midbody, leaving the head and long abdomen exposed. At the front of the carapace is a beaklike projection, or rostrum (RAH-strem), that extends out over the head.

The head has red compound eyes set on stalks. The first pair of antennae is usually branched, or biramous (BY-ray-mus). The second pair of antennae is uniramous (YU-neh-RAY-mus) and not branched. The mandibles (MAN-dih-bulz), or biting mouthparts, are uniramous. Maxillipeds (mack-SIH-leh-pe-hds), the leglike appendages associated with the mouth, are absent. The thorax and abdomen are distinct. The thorax has eight pairs of leaflike limbs that are all similar to one another in appearance. The seven-segmented abdomen or tail section has six pairs of limbs called pleopods (ple-o-pawds). The first four pairs are biramous. Each pair is hooked together so that they work together when swimming. The last two pairs of pleopods are small and uniramous. The tail segment at the tip of the abdomen is tipped with a long, forked projection.