The Sporozoa are widely distributed, occurring in various tissues and organs of Annelids, Molluscs, Arthropods, and Vertebrates. Their food, which is fluid, is absorbed osmotically. The life-cycle of a Sporozoön may be completed within one host or may be distributed between two different hosts. The Sporozoa were divided by Schaudinn into two groups or sub-classes, called (1) the *Telosporidia*, and (2) the *Neosporidia*. The Telosporidia are Sporozoa in which the reproductive phase of the parasites is distinct from the growing or trophic phase, and follows after it. The Neosporidia include Sporozoa in which growth and spore-formation go on simultaneously. This classification is not final, for certain exceptions and difficulties are already known with regard to it. It is possible that the class Sporozoa is not a natural entity, but should be replaced by two classes of equal rank, corresponding in most respects with the Telosporidia and Neosporidia. The *Telosporidia* comprise the *Gregarinida*, the *Coccidiidea*, and the *Hæmosporidia*. Doflein combines the two latter orders into one known as the *Coccidiomorpha*. The *Neosporidia* comprise the *Myxosporidia*, the *Microsporidia*, the *Actinomyxidia*, the *Sarcosporidia*, and the *Haplosporidia*. Doflein combines the first three orders into one, the *Cnidosporidia*. Sub-Class. TELOSPORIDIA, Schaudinn. Sporozoa in which the reproductive phases follow completion of growth. Order. *Gregarinida*, Aimé Schneider emend. Doflein. Knowledge of the Gregarinida probably goes back as far as the year 1684, when Redi observed gregarines in the crab, _Cancer pagurus_. Von Cavolini (1787) found them in _Cancer depressus_. The name _Gregarina_ was created by L. Dufour (1828), who observed masses of these organisms in the gut of insects of different orders. Hammerschmidt (1838) and von Siebold found rich infestations in insects, while Dujardin (1835) and Henle described various genera from segmented worms. Henle (1835) also observed cysts containing “navicellæ” in the sperm-sacs of segmented worms, and attention was drawn to his researches by the discovery by von Siebold (1839) of “pseudonavicellæ” in the gut of _Sciara nitidicollis_. Up to this time many workers considered the gregarines to be worms, but Kölliker (1845) investigated many of them and maintained their unicellular nature, while Stein’s work (1848) showed the interrelation of the pseudonavicellæ and the gregarines. The discovery of amœboid germs in the pseudonavicellæ by Lieberkühn (1855) and the demonstration of myonemes further aided in the elucidation of their true systematic position. The entire process of conjugation, of which Dufour had seen one phase, was followed by Giard under the microscope.