Each spore contains two (figs. 94, 95) or more polar capsules which are clearly visible in the fresh condition. Each polar capsule is a hollow, more or less pear-shaped body, secreted by a cell and having a well defined contour. Within it, a long, delicate, elastic filament, the polar filament, is formed, and lies spirally coiled in the polar capsule until just before the emergence of the amœbula from the spore (fig. 95). The polar filament is ejected, probably under the influence of the digestive juice, when the spore reaches a new host, and serves to anchor the spore to the tissue with which it is in contact, and thus allow of the emergence of the amœbula in a situation suitable for its development. The polar capsule with its contained polar filament has been compared with the stinging cells or nematocysts of the Cœlentera, but it has a totally different function. The spores fulfil the purpose of effecting transmission to other hosts. Infection occurs by the ingestion of the parasites per os after their escape by some means from their host. Thélohan and others have demonstrated that the valves of the spores soon open under the influence of the digestive juices, thus allowing the young myxosporidia to escape. Their further history is unknown; but it may be surmised that they either travel direct to the organs usually affected (gall bladder, urinary bladder), or are distributed in the body by means of the circulatory or lymphatic systems. The Myxosporidia that invade tissues are often deadly to their hosts. They may be present in a state of “diffuse infiltration” when practically every organ of the body may be infected, as in barbel disease (due to _Myxobolus pfeifferi_). On the other hand, the parasites may be concentrated at one spot, when cysts, either large or small, are produced. Such cysts occur on the gills of many fishes. A few additional important pathogenic forms are _Myxobolus cyprini_, the excitant of “pockenkrankheit” of carp, and _Lentospora cerebralis_, parasitic in the skeleton of Salmonidæ and Gadidæ. The skeletons of the tail, fins and skull particularly are seats of infection, and from the skull the Lentospora can spread to the semicircular canals, resulting in loss of power to maintain its balance on the part of the fish. On this account the malady is termed “drehkrankheit.” Young fish are more particularly infected. _Myxobolus neurobius_ infects the spinal cord and nerves of trout. Myxosporidia are divided into two sub-orders--_Disporea_ and _Polysporea_--according to whether they form only two or several spores during their growth. The former include two genera limited to fishes, which are easily distinguishable by the shape of the spores: _Leptotheca_, Thél., with a rounded spore, and _Ceratomyxa_, Thél., with a very elongate spore. The larger number of genera belong to the _Polysporea_, which are divided into three families: