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 Historical. 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 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 Occurrence; mode of infection. 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.
| From Wasielewski, after Léger. |
| Fig. 2.—Cysts of a Coelomic Gregarine, in the body-cavity of a larva of Tipula. |
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 Habitat and effects on host. 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).
| From Lankester. | From Lankester, after various authors. |
| Fig. 3.—Porospora gigantea f, (E. van Ben.), from the intestine of the lobster. a, Nucleus. | Fig. 4. a-c, Trophozoites of Monocystis agilis. a and b, Young individuals showing changes of body-form. c, Older individual, still enveloped in a coat of spermatozoa. d, e, Trophozoites of M. magna attached to seminal funnel of Lumbricus. Goblet-shaped epithelial cells, in which the extremity of the parasite is inserted. |
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 Coccidia). 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.
| After Siedlecki, from Lankester’s Treatise on Zoology. |
| Fig. 5.—Part of a section through the apparatus of fixation of a Pterocephalus, showing root-like processes extending from the Gregarine between the epithelial cells. g, Head of Gregarine; r, Root-like processes; ep, Epithelial cells. |
| From Wasielewski, after Léger. |
| Fig. 6.—Corycella armata, Léger. a, Cephalont; b, Epimerite in host-cell; c, Sporont. |
The body of a Gregarine is always of a definite shape, usually oval or elongated; in one or two instances (e.g. Diplodina) it is spherical, and, on the other hand, in Porospora (fig. 3) it is greatly drawn out and vermiform. In many adult Gregarines, Morphology. the body is divided into two distinct but unequal regions or halves, the anterior part being known as the protomerite, the hinder, generally the larger, as the deutomerite. This feature is closely associated with another important morphological character, one which is observable, however, only during the earlier stages of growth and development, namely, the presence of a definite organ, the epimerite, which serves for the attachment of the parasite to the host-cell (fig. 6).
In those Gregarines (most intestinal forms) which become attached to an epithelial cell, the attachment occurs by means of a minute projection or beak (rostrum) at the anterior end of the sporozoite, which pushes its way into the cell, followed by the first part of the growing germ. This portion of the body increases in size much quicker at first than the rest (the extracellular part), more or less fills up the host-cell, and forms the well-developed epimerite or secondary attaching organella. The extracellular part of the Gregarine next grows rapidly, and a transverse septum is formed at a short distance away from (outside) the point where the body penetrates into the cell (fig. 6); this marks off the large deutomerite posteriorly (distally). Léger thinks that this partition most likely owes its origin to trophic considerations, i.e. to the slightly different manner in which the two halves of the young parasite (the proximal, largely intracellular part, and the distal, extracellular one) may be supposed to obtain their nutriment. In the case of the one half, the host-cell supplies the nutriment, in that of the other, the intestinal liquid; and the septum is, as it were, the expression of the conflicting limit between these two methods. Nevertheless, the present writer does not think that mechanical considerations should be altogether left out of account. The septum may also be, to some extent, an adaption for strengthening the body of the fixed parasite against lateral thrusts or strains, due to the impact of foreign bodies (food, &c.) in the intestine.