All of the free-living cyclopoida are almost identical to each other in physical appearance. Their antennae are shorter than those of the calanoids, growing about half of the length of their bodies. Their bodies, relatively pair-shaped, have two clearly divided regions: the head and thorax in the front; and the last segment of the thorax fused with the abdomen in the rear. Their front portions narrow slowly into their abdomens. This order contains many marine forms and numerous freshwater representatives. They are rarely planktonic, rather they tend to swim near the bottom of the water, never migrating upwards. They thrive in small pools of water with large amounts of aquatic vegetation. Some of the larger species are carnivores, eating insects and other crustaceans.
There are also 12 or more families living in relation to other animals: either as hosts to parasites or as parasites themselves. Some freshwater species are important as temporary hosts to certain forms of worms that are parasitic to man. Other species are parasites on mollusks, sea anemones, or sea squirts. One specific group of parasitic cyclopoids live in the mouths or on the gills of certain fish, like frog-mouths. While the female can grow to 0.8 in (2 cm) long, the male never surpasses about 0.04 in (0.1 cm). The jaws of the female are shaped like sickles, enabling her to cling to her host and eat it.