Page:EB1911 - Volume 09.djvu/411

Rh parasites. Gen. Sphaerospora. Four or five species are known, from the kidneys or gall bladder of fishes (fig. 3, A). One, S. elegans, is interesting in that it affords a transition between the two sections, being disporous. Gen. Myxidium; spores elongated and fusiform, with a polar capsule at each extremity. The best-known species is M. lieberkühnii, from the urinary bladder of the pike. One or two species occur in reptiles. Other genera are Sphaeromyxa, Cystodiscus, Myxosoma and Myxoproteus.

Family, Chloromyxidae. Spores with four polar capsules and no iodinophilous vacuole. One genus, Chloromyxum, of which several species are known; the type being C. leydigi, from the gall bladder of various Elasmobranchs (fig. 7, B).

Family, Myxobolidae. Spores with two polar-capsules (exceptionally one), and with a characteristic iodinophilous vacuole in the sporoplasm. Typically tissue parasites of Teleosteans, often very dangerous. Genus Myxobolus. Spores oval or rounded, without a tail-like process. Very many species are known, which are grouped into three subsections: (a) forms with only one polar-capsule, such as M. piriformis, of the tench; (b) forms with two unequal capsules, e.g. M. dispar from Cyprinus and Leuciscus; and (c) the great majority of species with two equal polar-capsules, including M. mülleri, the type-species, from different fish, M. cyprini and M. pfeifferi, the cause of deadly disease in carp and barbel respectively and others. Other genera are Henneguya and Hoferellus, differing from Myxobolus in having, respectively, one or two tail-like processes to the spore. Lentospora, according to Plehn (37), lacks an iodinophilous vacuole.

Family Coccomyxidae. The pansporoblasts produce (probably) only one spore. Spore oval, large (14 by 5.5 ), with a single very large polar-capsule. Sporoplasm with no vacuole. Single genus Coccomyxa, with the characters of the family. One species, C. morovi, Léger and Hesse, from the gall bladder of the sardine. The spore greatly resembles a Cryptocystid spore.

Suborder 2: Cryptocystes, Gurley (= Microsporidia, Balbiani). Spores minute, usually pear-shaped, with only one polar-capsule, which is visible only after treatment with reagents. The number of spores formed in each pansporoblast varies greatly in different forms.

Section (a): Polysporogenea. The trophozoite produces numerous pansporoblasts, each of which gives rise to many spores. Genus Glugea, with numerous species, of which the best-known is G. anomala, from the stickleback (fig. 1). The genus Myxocystis, which has been shown by Hesse (24) to be a true Microsporidian, is placed by Perez in this section, but this is a little premature, as Hesse does not describe the exact character of the sporulation, i.e. with regard to the number of pansporoblasts and the spores they produce.

Section (b): Oligosporogenea. The trophozoite becomes itself the (single) pansporoblast. In Pleistophora, the pansporoblast produces many spores; P. typicalis, from the muscles of various fishes (fig. 2), is the type-species. In Thelohania, eight spores are formed; the different species are parasitic in Crustacea. In Gurleya, parasitic in Daphnia, only four are formed; and, lastly, in Nosema (exs. N. pulvis, from Carcinus, and, most likely, N. bombycis, of the silkworm), each pansporoblast produces only a single spore.

2. Order—Actinomyxidia. This order comprises a peculiar group of parasites, first described by A. Stolc in 1899, which are restricted to Oligochaete worms of the family Tubificidae. Most forms attack the intestinal wall, often destroying its epithelium over considerable areas; but one genus, Sphaeractinomyxon, inhabits the body-cavity of its host. The researches of Caullery and Mesnil (10-12) and of Léger (28 and 29) have shown that the parasites exhibit the typical features of the Endospora, and the spores possess the characteristic polar-capsules of the Myxosporidian spore, but differ therefrom by their more complicated structure.

The growth and development of an Actinomyxidian have been recently worked out by Caullery and Mesnil in the case of Sphaeractinomyxon stolci. A noteworthy point is the differentiation of an external (covering) cellular layer, which affords, perhaps, the nearest approach to distinct tissue-formation known among Protozoa. This envelope is formed soon after nuclear multiplication of the young trophozoite has begun, and is constituted by two nuclei and a thin, peripheral layer of cytoplasm. It remains binuclear throughout the entire period of development, and serves as a delicate cyst-membrane. The multiplication of the internal nuclei is accompanied by a corresponding division of the cytoplasm; so that instead of a multinucleate or plasmodial condition, distinct uninucleate cellules are formed, up to sixteen in number. These cellules, as a matter of fact, are sexual elements or gametes; and eight of them can be distinguished from the other eight by slight differences in the nuclei. The gametes unite in couples, each couple being most probably composed of dissimilar members: in other words, conjugation is slightly anisogamous. Each of these eight copulae gives rise to a spore.

As the name of the order implies, there are always eight spores formed. These differ from other Endosporan spores in having invariably a ternary symmetry and constitution (fig. 9). The wall of the spore is composed of three valves, each formed from an enveloping cell, and three capsular cells, placed at the upper or anterior pole, and containing each a polar-capsule, visible in the fresh condition. The valves are usually prolonged into processes or appendages, whose form and arrangement characterize the genus; but in Sphaeractinomyxon the spore is spherical and lacks processes. The sporoplasm may be either a plasmodial mass, with numerous nuclei, or may form a certain number of uninuclear sporozoites. A remarkable feature in the development of the spore is that the germinal tissue (sporoplasm) arises separate from and outside the cellules which give rise to the spore-wall; later, when the envelopes are nearly developed, the sporoplasm penetrates into the spore.

Four genera have been made known. (1) Hexactinomyxon, Stolc. Spores having the form of an anchor with six arms; sporoplasm plasmodial, situate near the anterior pole of the spore. One sp. H. psammoryctis, from Psammoryctes. (2) Triactinomyxon, St. Spores having the form of an anchor with three arms; distinct sporozoites, disposed near the anterior pole. T. ignotum, with eight spores, from Tubifex tubifex, and also from an unspecified Tubificid; another sp., unnamed, with 32 sporozoites, also from T. t. (3) Synactinomyxon, St. Spores united to one another, each having two aliform appendages; sporoplasm plasmodial. One sp., S. tubificis, from T. rivulorum. (4) Sphaeractinomyxon, C. and M. Spores spherical, without aliform prolongations; sporoplasm gives rise to very many