Copepods

Given the incredible number of species, the physical structure of copepods varies greatly. However, the free-living forms of copepods have certain physical traits in common. For instance, the body is usually short and cylindrical, composed of a head, thorax, and abdomen. The lower part of the copepod's head is generally fused with its thorax; the front of its head often juts forward, like a tiny beak. Its thorax is divided into about six segments; each segment is connected to two appendages.

Generally, a free-living copepod has two pair of antennae and a single eye. The first pair of antennae is larger and has bristles. The male copepod can be distinguished from female because its antennae are slightly different from those of the female, modified for holding her during copulation. The free-living copepods' limbs are used for movement, sometimes with the help of the antennae. Its thin abdomen lacks limbs, except for the caudal furca—an appendage akin to a tail. Its tail has bristles similar to those found on its primary antennae. Some tropical forms of copepods actually use their bristles to facilitate flotation.