| A. Telosporidia.—Cells 1-nucleate until the onset of brood-formation, which is simultaneous. | |
| 1. Gregarinidaceae.—Cells early provided with a firm pellicle and possessing a complex ectosarc; at first intracellular, soon becoming free in the gut or coelom of Invertebrates. Pairing between adults, which simultaneously produce each its brood of gametes, isogamous or bisexual, which pair within the common cyst; zygotospores surrounded by a firm cyst, and producing within a brood of sickle-shaped zoospores. | |
| (i.) Schizogregarinidae.—Multiplying by simple fission in the free state as well as by brood-formation; the brood-cells conjugating in a common cyst, but producing only one pairing nucleus in each mate (the rest aborting), and consequently only one spore. | |
| Ophryocystis A. Schn. | |
| (ii.) Acephalinidae.—Cell one-chambered, usually without an epimerite for attachment. | |
| Monocystis F. Stein; Lankesteria Mingazzini. | |
| (iii.) Dicystidae.—Cell divided by a plasmic partition; epimerite usually present. | |
| Gregarina Dufour; Stylorhynchus A. Schn.; Pterocephalus A. Schn. | |
| 2. Coccidiaceae.—Cells of simple structure, intracellular in Metazoa. Pairing between isolated cells usually sexually differentiated as oosphere and sperm, the latter often flagellate. Brood-formation of the adult cell giving rise to sickle-shaped zoospores (merozoites), or progamic and producing the gametes. Oosperm motile or motionless, finally producing a brood of spores, which again give rise to a brood of sickle-spores. | |
| (i.) Coccidiidae.—Cell permanently intracellular, or very rarely coelomic, encysting or not before division; zoospores always sickle-shaped; oosperm encysting at once, producing spores with a dense cell-wall producing sickle-germs. | |
| (ii.) Haemosporidae.—Cells parasitic in the blood corpuscles or free in the blood of cold-blooded animals, encysting before brood-formation; zoospores sickle-shaped; oosperm at first motile. | |
| Lankesterella Labbé; (Drepanidium Lank.;) Karyolysus Labbé; Haemogregarina Danilewski. | |
| (iii.) Acystosporidae.—Cells parasitic in the blood and haematocytes of warm-blooded Vertebrates; never forming a cyst-wall before dividing; zoospores formed in the corpuscles, amoeboid. Gametocytes only forming gametes when taken into the stomach of insects. Oosperm at first active, passing into the coelom, producing naked spores which again produce a large brood of sickle zoospores, which migrate to the salivary gland, and are injected with the saliva into the warm-blooded host. | |
| Haemamoeba Grassi and Feletti; Laverania Grassi and Feletti; Haemoproteus Kruse; Halteridium Labbé.[[104]] | |
| B. Neosporidia.—Cells becoming multinucleate apocytes before any brood-formation occurs. Brood-formation progressive through the apocyte, not simultaneous. | |
| 1. Myxosporidiaceae.—Naked parasites in cold-blooded animals. Spore-formation due to an aggregation of cytoplasm around a single nucleus to form an archespore, which then produces a complex of cells within which two daughter-cells form the spores and accessory nematocysts. | |
| Myxidium Bütsch.; Myxobolus Bütsch.; Henneguya Thélohan; Nosema Nageli (= Glugea Th.). | |
| 2. Actinomyxidiaceae.[[105]]—Apocyte resolved into a sporange, containing eight secondary sporanges (so-called spores), of ternary symmetry and provided with three polar nematocysts. | |
| 3. Sarcosporidiaceae.—Encysted parasites in the muscles of Vertebrates, with a double membrane; spores simple. | |
| Sarcocystis Lankester. | |
Fig. 32.—Gregarina blattarum Sieb. A, two cephalonts, embedded by their epimerite (ep), in cells of the gut-epithelium; deu, deutomerite; nu, nucleus; pr, protomerite; B1, B2, two free specimens of an allied genus; the epimerite is falling off in B2, which is on its way to become a sporont; C, cyst (cy) of A, with sporoducts (spd) discharging the spores (sp), surrounded by an external gelatinous investment (g). (From Parker and Haswell.)
Monocystis offers us the simplest type of Gregarinidaceae. In most Gregarines (Figs. 31, 32) the sporozoite enters the epithelium-cell of the gut of an Arthropod, Worm or Mollusc, and as it enlarges protrudes the greater part of its bulk into the lumen, and may become free therein, or pass into the coelom. The attached part is often enlarged into a sort of grapple armed with spines, the "epimerite"; this contains only sarcocyte, the other layers being absent. The freely projecting body is usually divided by an ingrowth of the myocyte into a front segment ("protomerite"), and a rear one ("deutomerite"), with the nucleus usually in the latter. In this state the cell is termed a "cephalont." Conjugation is frequent, but apparently is not always connected with syngamy or spore-formation; sometimes from two to five may be aggregated into a chain or "syzygy." The number of cases in which a syngamic process between two cells has been observed is constantly being increased. In Stylorhynchus (Fig. 33) the conjugation at first resembles that of Monocystis, but the actual pairing-cells are bisexually differentiated into sperms in the one parent, and oospheres in the other; it is remarkable that here the pear-shaped sperms are apparently larger than the oospheres. In Pterocephalus the chief difference is that the sperms are minute.[[106]] In all cases of spore-formation the epimerite is lost and the septum disappears; in this state the cell is termed a sporont. Sometimes the epiplasm of the sporont forms tubes ("sporoducts"), which project through the cyst-wall and give exit to the spores, as in Gregarina (Fig. 32, C), a parasite in the beetle Blaps.
Gregarines infest most groups of Invertebrates except Sponges and perhaps Coelenterates, the only exception cited being that of Epizoanthus glacialis, a Zoantharian (p. [406]). They appear to be relatively harmless and are not known to induce epidemics.
The Coccidiaceae never attain so high a degree of cellular differentiation as the Gregarines, which may be due to their habitat; for in the growing state they are intracellular parasites. Their life-history shows a double cycle, which has been most thoroughly worked out in Coccidiidae by Schaudinn and Siedlecki in parasites of our common Centipedes. We take that of Coccidium schubergi (in Lithobius forficatus[[107]]), beginning with the sporozoite, which is liberated from the spores taken in with the food, in the gut of the Centipede. This active sickle-shaped cell (Fig. 34, l) enters an epithelial cell of the mid-gut, and grows therein till it attains its full size (a), when it is termed a "schizont"; for it segments (Gk. σχίζω, "I split") superficially into a large number of sickle-shaped zoospores, the "merozoites" (c), resembling the sporozoites. The segmentation is superficial, so that there may remain a large mass of residual epiplasm. The merozoites are set free by the destruction of the epithelium-cell in which they were formed, and which becomes disorganised, like the residual epiplasm. Each merozoite may repeat the behaviour of the sporozoite, so that the disease spreads freely, and becomes acute after several reinfections. After a time the adult parasites, instead of becoming schizonts and simply forming merozoites by division, differentiate into cells that undergo a binary sexual differentiation. Some cells, the "oocytes" (d, e), escape into the gut, and the nucleus undergoes changes by which some of its substance (or an abortive daughter-nucleus) is expelled to the exterior (f), such a cell is now an "oogamete" or oosphere. Others, again, are spermatogones (h): each when full grown on escaping into the gut commences a division (i, j), like that of the schizonts. The products of this division or segment-cells are the flagellate sperms (s): they are more numerous and more minute than the merozoites produced by the schizonts, and are attracted to the oosphere by chemiotaxy (p. [23]), and one enters it and fuses with it (g). The oosperm, zygote or fertilised egg, thus formed invests itself with a dense cyst-wall, as a "oospore" (k), its contents form one or more (2, 4, 8, etc.) spores; and each spore forms again one, two, or four sickle-shaped zoospores ("sporozoites"), destined to be liberated for a fresh cycle of parasitic life when the spores are swallowed by another host.
Fig. 33.—Bisexual pairing of Stylorhynchus. a, Spermatozoon; b-e, fusion of cytoplasm of spermatozoon and oosphere; f, g, fusion of nuclei; h-j, development of wall to zygote; k, l, formation of four sporoblasts; l, side view of spore; m, mature sporozoites in spore. (After Léger.)
Fig. 34.—Life-history of Coccidium schubergi. a, Penetration of epithelium-cell of host by sporozoite; b-d, stages of multiple cell-formation in naked state (schizogony); e, f, formation of oogamete; g, conjugation; h-j, formation of sperms (s); k, development of zygote (fertilised ovum) to form four spores; l, formation of two zoospores (or sickle germs) in each spore. (From Calkins's Protozoa, after Schaudinn.)