1. Order Myxosporidia. The Myxosporidia, or, more correctly, the dense masses formed by their spores, were well known to the earlier zoological observers. The parasites in fishes were called by Müller “fish-psorosperms,” a name which has stuck to them ever since, although, as is evident from the meaning of the term (“mange-seed”), Müller had little idea of the true nature of the bodies. Other examples, infesting silkworms, have also long been known as “Pèbrine-corpuscles,” from the ravaging disease which they produce in those caterpillars in France, in connexion with which Pasteur did such valuable work. The foundation of our present morphological and biological knowledge of the order was well laid by the admirable researches of Thèlohan in 1895. In spite, however, of the contributions of numerous workers since then (e.g. Doflein, Cohn, Stempell and others), there are still one or two very important points, such as the occurrence of sexual conjugation, upon which light is required.
Although pre-eminently parasites of fishes, Myxosporidia also occur, in a few cases, in other Vertebrates (frogs and reptiles); no instance of their presence in a warm-blooded Vertebrate has, however, yet been described. One Occurrence and habitat. suborder (the Microsporidia or Cryptocystes) is pretty equally distributed between fishes on the one hand and Invertebrates—chiefly, but not exclusively, Arthropods—on the other. The parasites are frequently the cause of severe and fatal illness in their hosts, and devastating epidemics of myxosporidiosis have often been reported (e.g. among carp and barbel in continental rivers, due to a Myxobolus, and among crayfish in France, to Thelohania).
The seat of the invasion and the mode of parasitism are extremely varied. Practically any organ or tissue may be attacked, excepting, apparently, the testis and cartilage and bone. In one instance at least (that of Nosema bombycis of the silkworm) the parasites penetrate into the ova, so that true hereditary infection occurs, the progeny being born with the disease. The parasites may be either free in some lumen, such as that of the gall bladder or urinary bladder (not of the alimentary canal, or the body-cavity itself), when they are known as coelozoic forms; or in intimate relation with some tissue, intracellular while young but becoming intercellular in the adult phase (histozoic forms); or entirely intracellular (cytozoic forms). Among the histozoic and cytozoic types, moreover, two well-defined conditions, concentration and diffuse infiltration, occur. In the former, the parasitic zone is strictly limited, and well-marked cysts are formed; in the latter, the infection spreads throughout the neighbouring tissue, and the parasitic development becomes inextricably commingled with the host’s cells. Sometimes, as shown by Woodcock (45), there may be an attempt on the part of the host’s tissue to circumscribe and check the growth of these parasitic areas, which results in the formation of pseudocysts, quite different in character from true cysts.
| From Lankester’s Treatise on Zoology, vol. Protozoa, from Wasielewski, after Thélohan. | From Lankester’s Treatise on Zoology, vol. Protozoa. |
| Fig. 1.—Transverse section of a stickle-back (Gasterosteus aculeatus), showing two cysts of Glugea anomala, Moniez (kk), in the body musculature on the right side. | Fig. 2.—Portion of a section
through a muscle fibre of Cottus
scorpius invaded by Pleistophora
typicalis, Gurley. m, f, Muscle fibrils, retaining their striation. myx, Cysts of the parasite, lying between the fibrils. |
The most noticeable feature about the Myxosporidian trophozoite is its amoeboid and Rhizopod-like character. Pseudopodia of various kinds, from long slender ones (fig. 3, B) to short blunt lobose ones, are of general occurrence, being most easily observed, of Morphology. course, in the free-living forms. The pseudopodia serve chiefly for movement and attachment, and never, it should be noted, for the injection of solid food-particles, as in the case of Amoebae. The general protoplasm is divisible into ectoplasm and endoplasm. The former is a clear, finely-granular layer, of which the pseudopodia are mainly constituted (fig. 3, A). In one or two instances (e.g. Myxidium lieberkühnii) the ectoplasm shows a vertical striation, and in the older trophozoites breaks down partially, appearing like a fur of delicate, non-motile filaments. A somewhat similar modification is found in Myxocystis. The endoplasm is more fluid, and contains numerous inclusions of a granular nature, as well as vacuoles of varying size. In the endoplasm are lodged the nuclei, of which in an adult trophozoite there may be very many; they are all derived by multiplication from the single nucleus with which the young individuals begin life, the number increasing as growth proceeds.
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From Wasielewski, Sporozoenkunde. Fig. 3.—A. Trophozoite of Sphaerospora divergens, Thél. (par. Blennius and Crenilabrus),× 750. ec, Ectoplasm; en, endoplasm; sp, spores, each with four pole capsules. From Lankester’s Treatise on Zoology, vol. Protozoa. B. Spore-bearing trophozoite of Leptotheca agilis, Thél. (par. Trygon and Scorpaena),× 750. ps, Pseudopodia localized at the anterior end; f.gr, fatty granules similarlylocalized; r.gr, refringent granules; sp, spores, two in number. | |
Spore-formation goes on entirely in the endoplasm. The number of spores formed is very variable. It may be as low as two (as in free-living forms, e.g. Leptotheca), in which case a large amount of trophic protoplasm is unconverted Spore-formation; multiplicative processes. into spores; or, on the other hand, the number of spores may be very great (as in tissue-parasites), practically the whole of the parent-body being thus used up. The sporont may or may not encyst at the commencement of sporulation. In the free-living forms there is no cyst-membrane secreted; but in certain Glugeidae, on the other hand, the ectoplasm becomes altered into a firm, enclosing layer, the ectorind, which forms a thick cyst-wall (fig. 5). The process of sporulation begins by the segregation of small quantities of endoplasm around certain of the nuclei, to form little, rounded bodies, the pansporoblasts. There may be either very many or only few pansporoblasts developed; in some cases, indeed, there is only one, the sporont either itself becoming a pansporoblast (certain Microsporidia), or giving rise to a solitary one (Ceratomyxidae). The pansporoblast constituted, nuclear multiplication goes on preparatory to the formation of sporoblasts, which in their turn become spores (see figs. 4 and 5). Not all the nuclei thus formed, however, are made use of. In the Phaenocystes there are always two sporoblasts developed in each pansporoblast; in the Cryptocystes there may be from one to several. Around each sporoblast a spore-membrane is secreted, which usually has the form of two valves. It has recently been shown by Léger and Hesse (29b) that, in many Phaenocystes at any rate, each of these valves is formed by a definite nucleated portion of the sporoblast.
The spores themselves vary greatly in size and shape (figs. 7 and 8). They may be as small as 1.5 μ by 1 μ (as in a species of Nosema), or as large as 100 μ by 12 μ (as in Ceratomyxa). A conspicuous feature in the structure of a fully-developed spore is the polar-capsules, of which there may be either 1, 2, or 4 to each. In the Phaenocystes the polar-capsules are visible in the fresh condition, but not in the Cryptocystes. The polar-capsule is an organella which recalls the nematocyst of a Hydrozoan, containing a spirally-coiled filament, often of great length, which is shot out on the application of a suitable stimulus. Normally, as was ingeniously shown by Thélohan (43), the digestive juices of the fresh host serve this purpose, but various artificial means may suffice. The function of the everted filament is probably to secure the attachment of the spore to the epithelium of the new host. In the Phaenocystes, in connexion with each polar-capsule, a small nuclear body can be generally made out; these two little nuclei are those of the two “capsulogenous” areas of the protoplasm of the pansporoblast, which formed the capsules. The sporoplasm, representing the sporozoite, is always single. Nevertheless, in the Phaenocystes it is invariably binuclear; and, in the Microsporidia, the nucleus, at first single, gives rise later to four nuclei, two of which are regarded by Stempell (42) as corresponding to those of two polar-capsules (of which only one is developed in the spore), the remaining two representing germ-nuclei. Hence it is possible that the Myxosporidian sporoplasm really consists of two, incompletely-divided (sister) germs. Moreover, it is supposed by some that these two nuclei fuse together later, this act representing a sexual conjugation; since the earliest known phases of young trophozoites (amoebulae) have been described as uninuclear.
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| From Lankester’s Treatise on Zoology, vol. Protozoa, after Thélohan. | |
| Fig. 4.—Stages in spore-formation. All the figures are from Myxobolusellipsoides, except a and f, which are from M. pfeifferi. | |
a, Differentiation of the pansporoblast (p.sp). b, Pansporoblast with two nuclei. c and d, Pansporoblasts with sixand ten nuclei respectively;in d, four of the nuclei aredegenerating. e, Pansporoblast segmented intotwo definitive sporoblasts,each with three nuclei. Inthe next four figures the definitivesporoblast, or thespore produced from it, isalone figured. f, Definitive sporoblast segmentedinto three masses,the capsulogenous cells (c.g.c)and the sporoplasm (sp.p),within an envelope, the sporemembrane (sp.m). | g, More advanced stage. h, Spore completely developed,with two polar capsules andsporoplasm containing aniodinophilous vacuole. i, Abnormal spore containingsix polar capsules. n, Nuclei. sp.bl, Definitive sporoblast. r.n, Residuary nuclei. vac, Vacuole. r.p.c, Rudiment of p.c, polarcapsule. n.p.c, Nuclei of polar capsules. iod.vac, Iodinophilous vacuole. n.sp, Nuclei of sporoplasm. |
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| From Woodcock, Proc. and Trans. of the Liverpool Biological Society, 1904. | |
| Fig. 5.—Part of the periphery of a cyst of Glugea stephani, in theintestinal wall of the plaice, showing sporoblast and spore-formation. | |
ect, Ectorind. end, Endoplasm. endoth, Fold of the mucous membrane,normal in character. | p.sp.bl, Various stages in the developmentof the pansporoblasts. sp, Ripe spores, filling thegreater part of the cyst. n, Large (vegetative) nuclei. |