An order of the protozoan subclass Gregarinia, class Telosporea, subphylum Sporozoa. All gregarines are parasites of the digestive tract and body cavity of invertebrates or lower chordates; their large, mature trophozoites (vegetative stages) live outside the host's cells. The Archigregarinida are primitive gregarines, and live in marine worms (annelids and lower chordates—enteropneustids, sipunculids, and ascidians). Their life cycle includes sexual and asexual phases and involves three periods of schizogony (multiple fission).