[Illustration: FIG. 95.--Schematic representation of a spore of _Myxobolus_. One polar capsule has protruded its filament; two nuclei and a “vacuole” in the sporozoite. (After Doflein.)]
[Illustration: FIG. 96.--_Chloromyxum leydigi._ Active trophozoite (parasitic in gall-bladder of skates, rays, dog-fish). _Ect_, ectoplasm; _ps_, pseudopodia; _end_, endoplasm; _y_, yellow globules in endoplasm; _sp_, spores, each with four polar capsules. × 525. (After Thélohan.)]
The Myxosporidia live either free on the epithelial surface of hollow organs (gall or urinary bladder, renal tubules, but never in the intestine), or are enclosed in the tissues of their host. The gills and muscular system are their favourite habitat, but other tissues or organs may be attacked. Species of Myxosporidia are also known from Amphibia, Reptilia, and a few invertebrates.
The free forms, which are often amœboid (fig. 96), move by the aid of variously shaped pseudopodia, have a constant form, or may exhibit contractions of the body. The tissue parasites often reach a considerable size, so that the integument of the host forms protuberances over them. They are of a roundish or irregular shape. Frequently they are enveloped in a connective tissue covering formed by the host.
The protoplasmic body in the trophic phase (fig. 96) shows a distinct ectoplasm which is finely granular or sometimes striated, and an endoplasm which is coarsely granular and contains many nuclei as well as cell inclusions, such as crystals, pigment grains and fat globules. The nuclei originate by division from the primitive nucleus of the amœboid germ that issues from the spore. This amœbula may or may not live intra-cellularly during the early stages of its existence.
The multinucleate trophozoite of a Myxosporidian forms spores in its endoplasm practically throughout its whole period of growth (fig. 96). Vegetative reproduction by a process of external budding or plasmotomy may also occur, as in _Myxidium lieberkühni_ from the urinary bladder of the pike.
The myxosporidian trophozoite may produce two spores within itself, when it is placed in the sub-order _Disporea_, or it may produce numerous spores, which is characteristic of the sub-order, _Polysporea_. The phenomenon of spore formation is not simple (fig. 97), and the spore itself is surrounded by a bivalved shell or sporocyst and contains polar capsules in addition to the amœboid germ (fig. 97, G, H). The valves of the sporocyst and the polar capsules are really differentiated nucleate cells, so that each spore is an aggregate of cells rather than one cell, though only a single amœbula issues from a spore. The accounts of spore formation vary somewhat according to the different workers.