GREGARINES (mod. Lat. _Gregarina_, from _gregarius_, collecting in a flock or herd, _grex_) a large and abundant order of Sporozoa Ectospora, in which a very high degree of morphological specialization and cytological differentiation of the cell-body is frequently found. On the other hand, the life-cycle is, in general, fairly simple. Other principal characters which distinguish Gregarines from allied Sporozoan parasites are as follows:--The fully-grown adult (trophozoite) is always "free" in some internal cavity, i.e. it is extracellular; in nearly all cases prior to sporulation two Gregarines (associates) become attached to one another, forming a couple (syzygy), and are surrounded by a common cyst; inside the cyst the body of each associate becomes segmented up into a number of sexual elements (gametes, primary sporoblasts), which then conjugate in pairs; the resulting copula (zygote, definitive sporoblast) becomes usually a spore by the secretion of spore-membranes (sporocyst), its protoplasm (sporoplasm) dividing up to form the germs (sporozoites). Historical. F. Redi (1684) is said to have been the first to observe a Gregarine parasite, but his claim to this honour is by no means certain. Much later (1787) Cavolini described and figured an indubitable Gregarine (probably the form now known as _Aggregata conformis_) from a Crustacean (_Pachygrapsus_), which, however, he regarded as a tapeworm. Leon Dufour, who in his researches on insect anatomy came across several species of these parasites, also considered them as allied to the worms and proposed the generic name of _Gregarina_. The unicellular nature of Gregarines was first realized by A. von Kolliker, who from 1845-1848 added considerably to our knowledge of the frequent occurrence and wide distribution of these organisms. Further progress was due to F. Stein who demonstrated about this time the relation of the "pseudo-navicellae" (spores) to the reproduction of the parasites. [Illustration: From Wasielewswi's _Sporozoenkunde_, after Pfeiffer. FIG. 1.--a, Transverse Section of Intestine of Mealworm, infected with _Gregarina_ (_Clepsydrina_) _polymorpha_;[1] b, Part of a highly magnified.] Apart from the continually increasing number of known species, matters remained at about this stage for many years. It is, in fact, only since the closing years of the 19th century that the complete life-history has been fully worked out; this has now been done in many cases, thanks to the researches of M. Siedlecki, L. Cuenot, L. Leger, O. Duboscq, A. Laveran, M. Caullery, F. Mesnil and others, to whom also we owe most of our knowledge regarding the relations of the parasites to the cells of their host during their early development. Occurrence; mode of infection.