A group of entoparasites considered to be a subclass or order of the Trematoda. They have strongly developed ventral holdfasts whose alveolations are taxonomically important. Two families, Aspidogastridae and Stichocotylidae, are recognized. Aspidogastridae, which are the commonest, occur in various cavities of mollusks and in digestive tracts of fishes and turtles. Development is usually direct, involving nonciliated juveniles and one host. Fishes and turtles probably acquire infestations of adult worms by ingesting parasitized mollusks. Elongate Stichocotylidae have a single row of alveoli and occur in the digestive tracts of skates. Juvenile Stichocotyle nephrops encyst in lobsters and develop to maturity after ingestion by skates, thus approaching the digeneid life cycle. Little is known of the physiology of aspidogastreids, but they appear less host-specific than other trematodes.