A subclass of the class Telosporea. These protozoans occur principally as extracellular parasites in the digestive tracts and body cavities of invertebrates. Their spores are formed directly by the zygote. There are three orders: the Archigregarinida, whose life cycle embraces both sexual and asexual phases; the Eugregarinida, which increases only by sporogony; and the Neogregarinida, whose life cycle involves schizogony and gamont formation. The most familiar gregarines belong to the Eugregarinida and are represented by two types: cephaline, whose trophozites (sporadins) are divided into an anterior protomerite and a larger posterior deutomerite by a transverse septum; and acephaline, which lack the septum.