Monstrilloida is an order of copepods with a cosmopolitan distribution in the world's oceans. The order contains a single family, Monstrillidae; the family Thaumatopsyllidae was formerly included in the order, but is now usually placed in the Cyclopoida. The taxonomy of the family is undergoing a period of revision, although some necessary changes are apparent; for instance, the genus Strilloma is now considered a taxonomic synonym of Monstrilla, the largest genus.
The order is poorly known, biologically and ecologically, although the life cycle is known to differ from that of all other copepods. The larvae are parasites of benthic polychaetes and gastropods, while the adults are planktonic and incapable of feeding, functioning solely to reproduce.
The family Monstrillidae is characterised by having a well-developed fourth pair of swimming legs, but a rudimentary or absent fifth pair. Adults have no oral appendages, and the mouth leads only to a short, blind pharynx. Females carry a long pair of spines to which the eggs are attached, while males have a "genital protuberance, which is provided with lappets"; in both sexes, the genitalia are very different from those of all other copepods.
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