The Schizogregarines: A Review and a New Classification
Authors: Fantham H. B. 
Published: June 2019

1. The term Schizogregarinae Léger (1900) is the name now given to a sub-order of the Gregarinida, the remaining members of which are known as the Eugregarinae. The Schizogregarines were formerly known as Amoebosporidia Aimé Schneider (1884), a name given in misapprehension of the character of the cytoplasmic processes, fixative in function, present in the genus Ophryocystis. Two species of Ophryocystis (0. buetschlii and 0. francisci) were the only members of this sub-order known before 1900.2. At present the sub-order Schizogregarinae contains five families: Ophryocystidae, Schizocystidae, Selenidiidae, Merogregarinidae, and Aggregatidae.3. All these organisms show well-marked schizogonic stages in their life-history, and—with the possible exception of the Aggregatidae—follow after the Eugregarinae in their methods of sporogony.4. In Ophryocystis and Schizocystis the schizogony is extracellular, that is, these forms are ectoschizous. The life-cycle of the former is shown in Fig. 1. In these parasites the number of the nuclei in the schizont increases simultaneously with its volume.5. In Selenidium and Merogregarina the schizogony is intracellular, in other words these forms are endoschizous. The life-cycle of the former is illustrated in Fig. 3. In these forms the schizont is uninucleate during its growth, only becoming multinucleate at the end of the growing period.6. Ophryocystis forms only one sporocyst, a fact which has been emphasised by Léger and Duboscq (1908), by the placing of the Ophryocystidae in a special section, the Monospora. However, this apparent peculiarity is easily explained by a process of reduction and degeneration having taken place, affecting with one exception all the gametes formed from each gametocyte. There is good morphological evidence in support of this explanation (see p. 382, and Fig. 4, D).7. Figures of the interesting form Schizocystis gregarinoides (Léger, 1900) are not yet published, but a paper dealing with this organism is promised by Prof. Léger at an early date.8. Aggregata differs from other Schizogregarines in that its schizogony takes place in one host (crab), while its sporogony occurs in another (Cephalopod mollusc). In this respect Aggregata resembles the Haemosporidia. The schizogonic phases in Crabs were formerly regarded as belonging to a gymnosporous Gregarine, Aggregata Frenzel, while the sporogonic phases were considered to belong to a Coccidian, Eucoccidium (Benedenia) in cuttlefishes and Octopus. Regarding this, Léger and Duboscq (1908, p. 102) write “…Aggregata, avec un changement de cycle coïncidant avec un changement d'hôte, c'est à dire qui soient à la fois digénétiques et hétéroïques.”9. It is evident therefore that the Aggregatidae stand apart. On this account, I suggest a division of the Schizogregarinae into two sections, termed respectively, the Homoïica (to include the first four families discussed in this paper, wherein schizogony and sporogony take place in the same host) and the Heteroïca (for the Aggregatidae).10. Among the Homoïca we have extracellular schizogony (ectoschizous forms) in the Ophryocystidae and Schizocystidae, and intracellular schizogony (endoschizous forms) in the Selenidiidae and Merogregarinidae. This difference is not merely superficial, it requires to be emphasised, and for this reason I would divide the section Homoïca into two sub-sections, termed respectively Ectoschiza and Endoschiza.11. The classification of the Schizogregarines, which I would propose, is as follows:12. Much further research is needed on the life-cycles of the Endoschiza, especially among the Selenidiidae, which occur so frequently in the Annelida. Sporogonic stages are at present unknown in Eleutheroschizon and Siedleckia.13. In connection with the Aggregatidae, and to a less extent with the Selenidiidae, stress is laid upon the necessity of carefully distinguishing between “coelomic” and “gut” parasites. (See pp. 397 and 387.)14. The Schizogregarinae form a most interesting link between the Eugregarinae and the Coccidiidea.