Copepoda (phylum Arthropoda, sub-phylum Crustacea) Large and diverse class of mostly marine crustaceans, though there are many freshwater species and a few which live in water films between mosses and soil particles. Many are ectoparasites of fish and whales. In free-living forms, the body is usually short and cylindrical, the 10-segmented trunk comprising a thorax and an abdomen. The head may bear a pointed rostrum, or be rounded anteriorly. A median nauplius eye is typical of all copepods, as is the absence of compound eyes. The first antennae are uniramous, and well developed; the second pair are smaller. The first pair of thoracic appendages are feeding structures and are modified to form maxillipeds. With the exception of the last one or two pairs, the remaining thoracic appendages are similar, and symmetrically biramous. The posterior of the head is fused to the first thoracic segment and also, occasionally, to the second. The tapered thorax comprises three–five unfused segments with appendages. The narrow, cylindrical abdomen is devoid of appendages; the anal segment bears two caudal rami. Many copepods are planktonic, occur in vast numbers, and—as the major food of many marine animals—are the main link between phytoplankton and the higher trophic levels. Most species are tiny, ranging from less than one to several millimetres in length, though the species of Penellus, which are ectoparasites of fish and whales, may attain 32 cm or more. There are more than 7500 species.

Copepoda A class of crustaceans occurring in marine and freshwater habitats. Copepods are usually 0.5–2 mm long and lack both a carapace and compound eyes. Copepods are important members of plankton: some are free-living, feeding on microscopic organisms; others are parasitic. A familiar freshwater genus is Cyclops, so named because the members have a single median eye.