Gregarines are essentially parasites of Invertebrates; they are not known to occur in any true Vertebrate although met with in Ascidians. By far the greatest number of hosts is furnished by the Arthropods. Many members of the various groups of worms (especially the Annelids) also harbour the parasites, and certain very interesting forms are found in Echinoderms; in the other classes, they either occur only sporadically or else are absent. Infection is invariably of the accidental (casual) type, by way of the alimentary canal, the spores being usually swallowed by the host when feeding; a novel variation of this method has been described by Woodcock (31) in the case of a Gregarine parasitic in Cucumaria, where the spores are sucked up through the cloaca into the respiratory trees, by the inhalant current. [Illustration: From Wasielewski, after Leger. FIG. 2.--Cysts of a Coelomic Gregarine, in the body-cavity of a larva of _Tipula_.] Habitat and effects on host. The favourite habitat is either the intestine (fig. 1) or its diverticula (e.g. the Malpighian tubules), or the body-cavity. In the latter case, after infection has occurred, the liberated germs at once traverse the intestinal epithelium. They may come to rest in the connective tissue of the sub-mucosa (remaining, however, extracellular), grow considerably in that situation, and ultimately fall into the body-cavity (e.g. _Diplocystis_); or they may pass straightway into the body-cavity and there come into relation with some organ or tissue (e.g. _Monocystis_) of the earthworm, which is for a time intracellular in the spermatoblasts (fig. 4, c). In the case of intestinal Gregarines, the behaviour of the young trophozoite with respect to the epithelial cells of its host varies greatly. The parasite may remain only attached to the host-cell, never becoming actually intracellular (e.g. _Pterocephalus_); more usually it penetrates partially into it, the extracellular portion of the Gregarine, however, giving rise subsequently to most of the adult (e.g. _Gregarina_); or lastly, in a few forms, the early development is entirely intracellular (e.g. _Lankesteria_, _Stenophora_). [Illustration: From Lankester. FIG. 3.--_Porospora gigantea f_, (E. van Ben.), from the intestine of the lobster. a, Nucleus.] [Illustration: From Lankester, after various authors. FIG. 4. a-c, Trophozoites of _Monocystis agilis_. a and b, Young individuals showing changes of body-form. c, Older individual, still enveloped in a coat of spermatozoa. d, e, Trophozoites of _M. magna_ attached to seminal funnel of _Lumbricus_. Goblet-shaped epithelial cells, in which the extremity of the parasite is inserted.]