The bdelloids (Bdelloidea) are a class of rotifers, found in freshwater and moist soil. They typically have a well developed corona, divided into two parts, on a retractable head. They may move by swimming or crawling. The latter commonly involves taking alternate steps with the head and tail, as do certain leeches, which gives the group their name (Greek βδέλλα or bdella, meaning leech).
Bdelloids have been of interest to those interest in the evolutionary role of sexual reproduction, because it has disappeared entirely from the group: males are unknown, and females reproduce exclusively by parthenogenesis. Each individual has paired gonads.