The Copepoda is classified within the class Maxillopoda, on the basis of the body plan, which consists of five head segments, seven thoracic segments (of which the last bears the genital openings in both sexes), and a limb-less abdomen of four segments. The Copepoda is a large group, currently comprising nine orders, about 220 families, and about 13,500 species. The orders include:
• Platycopioida: a small order comprising just one family with three genera and about 12 species found in the near-bottom plankton community in coastal waters and in the plankton of flooded marine caves.
• Calanoida: the dominant order of planktonic copepods, comprising 42 families found from the surface to the greatest depths of the ocean, as well as in the plankton of freshwater lakes and ponds.
• Misophrioida: a small order comprising three families and a total of 32 species found in the near-bottom plankton community in marine waters and in the plankton of flooded marine caves.
• Mormonilloida: this order consists of two species in one family. Both are widely distributed in the deep-water plankton of the world's oceans.
• Harpacticoida: a large and diverse order of mainly bottom-living forms. A few of the 52 families are members of the marine plankton community, and some are even parasitic on animal hosts, but the great majority are benthic (bottom living), including forms that are interstitial (living in the spaces between sediment particles) and forms that live on the surface of the sediment or on macroalgae.
• Gelyelloida: this order consists of just two species in one family. Both are found only in groundwater in the karstic regions of southern Europe.
• Cyclopoida: a large and diverse group, comprising about 83 families exhibiting a wide range of lifestyles from planktonic to benthic, as well as symbiotic. The benthic forms include members of the family Cy-clopidae that have invaded groundwater habitats. The symbiotic forms include many that are parasitic on marine and freshwater fishes, as well as on a whole range of marine invertebrate hosts. In addition, some cyclopoid families are extremely abundant in marine zooplankton communities.
• Siphonostomatoida: all 37 families of this order have symbiotic lifestyles. They use as hosts virtually every phylum of multicellular animals from sponges up to vertebrates, including mammals (whales). Most are marine, but a few species from three families live in freshwater.
• Monstrilloida: this order consists of a single family containing about 80 species, all of which occur in marine zooplankton as adults, but have larvae living as endoparasities of worms and mollusks.
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