Amphipods: Amphipoda

Amphipods come in a wide variety of shapes and sizes, but most have long, c-shaped bodies that are flat from side to side. Amphipods usually measure 0.2 to 0.6 inches (5 to 15 millimeters) in length, but some deep-sea species reach up to 9.8 inches (250 millimeters).

All amphipods have bodies that are made up of three regions: head, thorax, and abdomen. Both pairs of antennae are well developed and unbranched, or uniramous (YU-neh-RAY-mus). The compound eyes, if present at all, are not set on stalks. Each compound eye has multiple lenses. The eight-segmented thorax has eight pairs of uniramous appendages. The first thoracic (thuh-RAE-sik) segment is tightly joined, or fused, to the head. Its appendages are called maxillipeds (mack-SIH-leh-pehds). Maxillipeds are thoracic limbs that work together with the mouthparts. The remaining seven pairs of thoracic limbs are called pereopods (PAIR-ee-oh-pawds). The first two pairs of pereopods end in pincherlike claws that are used for grasping. The last five pairs of pereopods are used for burrowing, crawling, and jumping. Respiratory organs, or gills, are also found on the thorax.

The six-segmented abdomen has three pairs of appendages called pleopods (PLEE-oh-pawds). The pleopods are used for swimming and for moving oxygen-carrying water through the burrow. Another three pairs of abdominal appendages are called uropods (YUR-oh-pawds). The uropods are used for burrowing, jumping, and swimming. At the end of the abdomen is a small, taillike segment called the telson. Depending on the species, the telson is sometimes fused to the last abdominal segment.