Page:EB1911 - Volume 12.djvu/581

 The unicellular nature of Gregarines was first realized by A. von Kölliker, 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.

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. Cuénot, L. Léger, 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.

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.

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).

The effects on the host are confined to the parasitized cells. These generally undergo at first marked hypertrophy and alteration in character; this condition is succeeded by one of atrophy, when the substance of the cell becomes in one way or another practically absorbed by the growing parasite (cf. also ). Since, however, the Gregarines never overrun their hosts in the way that many other Sporozoa do (because of their lack, in general, of the power of endogenous multiplication), the number of cells of any tissue attacked, even in the case of a strong infection, is only a very small percentage of the whole. In short the hosts do not, as a rule, suffer any appreciable inconvenience from the presence of the parasites.

The body of a Gregarine is always of a definite shape, usually oval