Amphionids: Amphionidacea

There is only one species of amphionid, Amphionides reynaudii. Adult males are unknown; females are about 1 inch (2.6 centimeters) long. Their bodies are divided into a head, thorax, and abdomen. The head and thorax are completely covered by a thin, membranelike carapace. There is a considerable amount of space between the carapace and the underside of the body. The head has a pair of compound eyes on stalks that are believed to produce light. Compound eyes have multiple lenses. Both pairs of antennae are branched, or biramous (BY-ray-mus). The second pair of antennae also has a single, large, leaflike flap on their bases.

The thorax has seven segments. Segments 3 and 7 have feathery gills. Juvenile males have seven pairs of thoracic limbs, but the last pair is absent in females. Pairs 1 through 6 are biramous. Only the first pair of limbs, the maxillipeds, is used for swimming. The remaining limbs are sticklike and useless for swimming. In females, the fifth pair of thoracic limbs is very long, and the sixth pair has the openings to the reproductive system. The last pair of thoracic limbs (juvenile males only) is unbranched, or uniramous (YU-neh-RAY-mus).

The abdomen is divided into six segments. The first five segments each has a pair of appendages underneath called pleopods (PLEE-oh-pawds). In females, the first pair of pleopods is long, ribbonlike, and uniramous. They reach toward the head and are about half the length of the carapace. They are used to close off the underside of the carapace to form a brood chamber when protecting eggs. The remaining four pairs of pleopods are much shorter and biramous. The tip of the abdomen has a pair of long appendages called uropods (YUR-oh-pawds). The uropods are found on either side of a flaplike tail, or telson. The uropods and telson together form a fanlike tail.