The Sporozoans known by this name are parasitic in various freshwater Fish and certain Elasmobranchii. The skin of the head and opercular cavities, the branchiae and viscera are the commonest localities; the muscles of the body are seldom infected, and the central nervous system is completely exempt. They are sometimes found in delicate cysts with nucleated walls, probably derived from the tissues of the host, ranging in size from about \ in. downwards, but the Myxosporidium of the Pike's urinary bladder lives free on the surface of the mucous membrane. The organism is Amoeba-like, more or less rounded or elongate; it shows a distinct division of exoplasm and endoplasm. The surface of the exoplasm may be covered completely or partially with fine, simple, or branched retractile processes. The endoplasm is granular, sometimes coloured with yellow or brown fatty granules, and crystals of haematoidin: it is multinucleate. Movement may be slowly effected by lobose pseudopodia. Spore-formation commences at an early period, and is perhaps a continuous phenomenon. The number of spores present is variable. Their development has been traced in the Myxosporidium inhabiting the Pike's bladder. Clear globules of protoplasm make their appearance in the endoplasm, containing six nuclei apiece.

A delicate membrane appears round each of them, and then each divides into two tri-nucleate halves, the spores, which become fusiform. Two of the nuclei disappear leaving one in the centre. The spore has no membrane and at either end of it a clear 'polar body' becomes visible. This body contains a long internal and spirally coiled thread, which is shot out on the application of alkalies, etc. It is therefore a trichocyst, and it is supposed that the discharged thread serves to anchor the spore to any soft body into which it may happen to penetrate. The spores of other Myxo-sporidia may be flattened and lens-like with one pole somewhat pointed, more rarely ovate, or fusiform, and pointed at each end. The spore-case is generally bivalved and drawn out into a simple or forked process. The pointed pole is always open, and trichocysts to the number of one, two, three, rarely four or eight, are lodged at the aperture. The escape of an amoebida from the spore-case has been observed.

There are two Sporozoans of doubtful position, Lithocystis Schneider!, and Amoebidium parasiticum. As to the former, it is found in the coelome of the Spatangid Echinoid, Echinocardium cordatum, on the inner aspect of the test and the outer of the intestine, in the shape of large irregular black masses, perhaps plas-modial in nature. They may attain a size of about 3/8 inch long, and 1/5 broad. Adherent to these black masses are a variable number of hyaline spheres 1/25-2/25 inch in size. Each sphere has a structureless membrane, one, rarely more, masses of crystals which are dispersed in later stages, and a number of spores. The latter are at first disposed round a yellowish spot or mass to which they adhere each by two filaments. During the course of development they become rearranged if the cyst is large enough and adhere by their other extremities, the filaments then looking like rigid flagella. The spore contains a granular protoplasm which is resolved into 3-6 falciform bodies, and a residual mass which becomes reduced to 2-3 granules or disappears completely. Giard, who discovered this organism, believes that the falciform bodies give origin to some of the amoebae, so common in the coelomic fluid.

See C. R. 82, 1876, or A. N. H. (4), xviii. 1876, p. 192.

Amoebidium parasiticum was discovered by Cienkowski. It is a tubular organism, about .5 mm. long and occurs attached by one end to the feet and branchiae of larval Phryganids and Libellulids, and certain freshwater Crustacea, Gammarus, Ase//us, etc. The tube has a delicate wall, finely granular protoplasm, often vacuolate, and containing a series of nuclei placed at regular intervals along the axis. The protoplasm segments, a portion to each nucleus, forming small fusiform bodies which escape and either grow into new Amoebidia, or their nuclei divide once or twice and their protoplasm segments, giving rise to amoebulae. The parent organism may produce similar amoebulae. The latter move about by means of lobose pseudopodia. They either form thick-walled resting cysts, the contents of which eventually break up into a number of fusiform bodies, either within the cyst or after escaping from it inclosed in a delicate membrane, or they form an oval thin-walled cyst the contents of which break up at once in the same manner.

By Cienkowski, Amoebidium was considered to be a fungal; Balbiani groups it with the Sarcosporidia. See Cienkowski, Botan. Zeitung, xix. 1861; Balbiani, op. cit. infra, p. 116; Butschli, op. cit. infra, p. 611.

The Gregarinida are classified by Butschli, op. cit. infra, as follows: I. Monocystidea. Body not septate.

(i) Coccidiidae: a provisional group distinguished from (ii) infra, by the fact that the various genera appear to be intra-cellular parasites throughout their whole lives, the cyst being extruded when it is fully formed: subdivided by Schneider into three sub-groups.

(1) Monosporea: the contents of the cyst constitute a single spore. Orthospora, from the intestinal epithelium of the Newt (Triton); the spore gives origin to four falciform bodies. Eimeria with three species, one from the intestine of the mouse, the kidneys of the horse, etc, a second from the intestine of Litho-bius, a third from the Malpighian vessels of Glomeris; the cyst has one to two micropyles, and the falciform bodies are produced by a process of one-sided gemmation.

(2) Oligosporea: the cyst gives origin to a few spores, constant in number. Cyclospora, two species, one from the intestine of Glomeris, the other from that of the cat; two spores with 2-4 falciform bodies apiece. Isospora, from Limax; two spores with many falciform bodies. Coccidium, sp. ? from various Vertebrata; cyst ovate with a micropyle, four spores, each with but one falciform body.

(3) Polysporea: the cyst gives origin to a number of spores with 2-4-6 falciform bodies apiece. Klossia with several species from the nephridia of Helix hortensis, Succinea amphibia, Neritina fluviatilis, and Chiton, and nearly every organ of Sepia and Octopus: from Blaps, Gyrinus, and Scolopendra morsitans.

(ii) Monocystidae: free in the unencysted condition. Several genera, e. g. Adelea, from intestine of Lithobius; Monocystis from the vesiculae seminales of Lum-bricus; Urospora from the Ascidian Ciona intestinalis, Tubifex, Sipunculus, various Nemerteans, etc.

II. Polycystidea

Body septate; rarely consisting of two segments only from the first (Gamocystis, Porospora (?)), usually of three, an epimerite, which is eventually cast off more or less completely, a protomerite, and deuteromerite. A large number of genera is known, e. g. Clepsidrina, Slylorhynchus: but their arrangement into families is uncertain at present. Schneider is treating of them in his Tablettes Zoologiques and promises a complete monograph.

The remaining sub-classes contain but few genera and do not need subdivision.

Biitschli, Bronn's Klass. und Ordn. des Thierreichs, i. Protozoa, pp. 479-616. Balbiani, Lecons sur les Sporozoaires, Cours d'Embryologie compare'e du College de France, Paris, 1884.

(i) Gregarinida. Coccidiidae: Schneider, A. Z. Expt. ix. 1881; Id. Klossia, op. cit. (2), i. 1883; Id. Tablettes Zoologiques, i. Poitiers, 1886, pp. 4, 88, cf. p. 104. Globidium (=Eimeria)), Flesch, Recueil Zool. Suisse, i. 1884. Gymnospora, Moniez, Bull. Soc. Zool. France, xi. 1886. New Sporozoa, Pachinger, Z. A. ix. 1886, p. 471. Cf. Leuckart, 'Parasites of Man,' transl. by Hoyle, i. Edinburgh, 1886, p. 202. Monocystidae nd Polycystidea, Schneider, A. Z. Expt. ii. 1873; iv. 1875; x. 1882; Id. Tablettes Zoologiques (supra), pp. 25, 90. Gregarines of Earthworm, Ruschhaupt, J. Z. xviii. 1885. Marine Gregarines, Frenzel, A. M. A. xxiv. 1885. Monocystid of Blatta Americana, Kunstler, C. R. 98, 1884. Neozygetis Aphidis, Witlaczil, A. M. A. xxiv. 1885. Gregarines of Phalangidae, Rossler, Z. W. Z. xxxvi. 1882, p. 700. Urospora of Ciona intestinalis, Parona, Journal de Micrographie, x. 1886: U. (Monocystis) Sipunculi, Ray Lankester, Q. J. M. xii. 1872. Conorhynchus Echiuri, Greeff, 'Die Echiuren,'p. 128, Nova Acta, 41, 1879. Haematozoalforms, note 1, p. 859 ante.

Amyloid granules, Maupas, C. R. 102, 1886; Butschli, Zeitschrift fur Biologie, xxi. 1885.

Development of Polycystids. Clepsidrina Blattarum, Butschli, Z. W. Z. xxxv. 1881. Stylorhynchus longicollis, Schneider, A. Z. Expt. x. 1882, p. 423; (2), ii. 1884; of various forms, Id. Tablettes Zoologiques (supra), pp. 10, 81; cf. p. 104.

(ii) Amoebosporidia: Ophryocystis, Schneider, A. Z. Expt. (2), ii. 1884, cf. A. N. H. (5), xiv. 1884; Id. Tablettes Zoologiques (supra), p. I.

(iii) Sarcosporidia: Blanchard, Bull. Soc. Zool. France, x. 1885; Leuckart, op. cit. supra, p. 199.

(iv) Myxosporidia: Butschli, 'Fischpsorospermien,' Z. W. Z. xxxv. 1881; Me*gnin, 'Role pathologique,' etc, Bull. Soc. Zool. France, x. 1885.