The Gregarines are extremely numerous, and include several families, characterized, for the most part, by the form of the spores (fig. 16). The specialized _Schizogregarinae_ are usually separated off from the rest as a distinct sub-order. SUB-ORDER I.--_Schizogregarinae._ Forms in which schizogonic reproduction is of general occurrence during the extra-cellular, trophic phase. Three genera, _Ophryocystis_, _Schizocystis_ and _Eleutheroschizon_, different peculiarities of which have been referred to above. Mostly parasitic in the intestine or Malpighian tubules of insects. (In this type of parasite, as exemplified by _Ophryocystis_, the body was formerly wrongly considered as amoeboid, and hence this genus was placed in a special order, the _Amoebosporidia_.) [Illustration: From Wasielewski, after Leger. FIG. 16.--Spores of various Gregarines. a, _Eirmocystis, Sphaerocystis_, &c. b, _Echinomera, Pterocephalus_, &c. c, _Gregarina_, &c. d, _Beloides_. e, _Ancyrophora_. f, _Stylorhynchidae_ (type of). g, _Menosporidae_. h, _Gonospora terebellae_. i, _Ceratospora_. k, _Urospora synaptae_.] SUB-ORDER II.--_Eugregarinae._ Schizogony very exceptional, only occurring during the intracellular phase, if at all. Gregarines fall naturally into two tribes, described as cephalont and septate, or as acephalont and aseptate (haplocytic), respectively. In strictness, however, as already mentioned, these two sets of terms do not agree absolutely, and whichever set is adopted, the other must be taken into account in estimating the proper position of certain parasites. Here the cephalont or acephalont condition is regarded as the more primary and fundamental. Tribe A.--_Cephalina_ (practically equivalent to _Septata_). Save exceptionally, the body possesses an epimerite, at any rate during the early stages of growth, and is typically septate. Mostly intestinal parasites of Arthropods. The chief families, with representative genera, are as follows: _Porosporidae_, with _Porospora gigantea_, at present thought to be gymnosporous; _Gregarinidae_ (_Clepsydrinidae_), with _Gregarina_, _Clepsydrina_, _Eirmocystis_, _Hyalospora_, _Cmenidospora_, _Stenophora_; _Didymophyidae_, with _Didymophyes_; _Dactylophoridae_, with _Dactylophorus_, _Pterocephalus_, _Echinomera_, _Rhopalonia_; _Actinocephalidae_ with _Actinocephalus_, _Pyxinia_, _Coleorhynchus_, _Stephanophora_, _Legeria_, _Stictospora_, _Pileocephalus_, _Sciadophora_; _Acanthosporidae_ with _Acanthospora_, _Corycella_, _Cometoides_; _Menosporidae_ with _Menospora_, _Hoplorhynchus_; _Stylorhynchidae_, with _Stylorhynchus_, _Lophocephalus_; _Doliocystidae_ with _Doliocystis_; and _Taeniocystidae_, with _Taeniocystis_. The curious genus _Selenidium_ is somewhat apart. Tribe B.--_Acephalina_ (practically equivalent to _Aseptata_, _Haplocyta_).