COPEPODS

A common feature uniting all the copepod orders is a single simple eye in the middle of the head, at least in the larval stage. The cephalosome, a shield over the head and some thoracic segments distinguishes the free living forms. Most are very small, less than 1 mm long, but rare oceanic species are over 1 centimetre.

There are several different forms of copepods arranged into ten groups called orders. It is not a simple matter to distinguish the orders of copepods, and impossible without a good microscope. Few biologists attempt to identify copepods beyond the level of order but species diversity has been found by those that have to be very high. Most of the free-living species belong to only three orders. The rest are usually parasitic, some barely recognisable as crustaceans except when larvae.

These aquatic crustaceans are very diverse and are the most numerous metazoans in the water community ("metazoan" means all multi-celled animals).  Copepod habitats range from fresh water to hypersaline conditions, from subterranean caves to water collected in bromeliad leaves or leaf litter on the ground and from streams, rivers, and lakes to the sediment layer in the open ocean.  Their habitats also range from the highest mountains to the deepest ocean trenches, and from the cold polar ice-water interface to the hot active hydrothermal vents.  Copepods may be free-living, symbiotic, or internal or external parasites on almost every phylum of animals in water.  The usual length of adults is 1-2 mm, but adults of some species may be as short as 0.2mm and others may be as long as 10mm. 

Ecologically they are important links in the food chain linking microscopic algal cells to juvenile fish to whales. Copepods also have the potential to act as control mechanisms for malaria by consuming mosquito larvae, and contrariwise are intermediate hosts of many human and animal parasites.

Branchiura (commonly referred to as sea lice) are also included with the Copepoda, since many copepod researchers also study these parasites of fish.

The Subclass Copepoda is comprised of 9 Orders:
Calanoida		Cyclopoida
Gelyelloida		Harpacticoida
Misophrioida		Monstrilloida
Mormonilloida		Platycopioida
Siphonostomatoida
There are over 210 described families, 2,400 genera and 24,000 described species (both valid and invalid, including senior and junior synonyms)
Poecilostomatoida [Boxshall & Halsey, 2004: 37, placed it into Cyclopoida]
The Class Branchiura is comprised of 1 Family and 4 Genera
Argulus	Chonopeltis	Dipteropeltis	Dolops

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