|Striped fish louse
Isopods are an order of peracarid crustaceans, including familiar animals such as woodlice and pill bugs. The name Isopoda derives from the Greek iso meaning "same" and pod meaning "foot"
Isopods are relatively small crustaceans with seven pairs of legs of similar size and form, ranging in size from 300 micrometres to nearly 50 centimetres in the case of Bathynomus giganteus . They are typically flattened dorso-ventrally, although many species deviate from this plan, particularly those from the deep sea or from ground water . Isopods lack an obvious carapace, which is reduced to a "cephalic shield" covering only the head . Gas exchange is carried out by specialised gill-like pleopods towards the rear of the animal's body. In terrestrial isopods, these are often adapted into structures which resemble lungs.
A number of isopod groups have evolved a parasitic lifestyle. The suborder Epicaridea is exclusively parasitic, while the polyphyletic suborder Flabellifera is partly parasitic. Cymothoa exigua, for example, is a parasite of the spotted rose snapper fish Lutjanus guttatus in the Gulf of California; it eats the tongue of the fish, and takes its place, in the only known instance of a parasite functionally replacing a host structure
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