[Illustration: FIG. 93.--Upper figure, part of a gill of a roach, _Leuciscus rutilus_ (natural size), with two myxosporidia. Lower figures, _a_, _b_, _d_, spores of myxosporidia from a pike, _Esox lucius_. _c_, Spore from _Platystoma fasciatum_. (After J. Müller.)] [Illustration: FIG. 94.--The tailless spore of _Myxobolus mülleri_, with the polar bodies and their nuclei and the sporozoite. (After Bütschli.)] These parasites, which were discovered by Johannes Müller (1841), live principally in fishes, and occasionally cause destructive epizoötics amongst their hosts. Müller first observed them in the form of whitish-yellow pustules on the skin or on the gills of various fishes. These pustules contained masses of small shell-covered bodies with or without tails (“psorosperms,” see fig. 93). Similar bodies were also found in the air bladders of certain fish. Creplin (1842) demonstrated the resemblance of the cysts (“psorosperm tubes”) harbouring the psorosperms to the “pseudonavicella-cysts” of a gregarine, as described by v. Siebold. Dujardin (1845) considered that there was possibly some connection between the protoplasmic “psorosperm tubes” and the spores they contained, and the developmental stages of monocystid gregarines from the vesiculæ seminales of earth-worms. The relationship of the “fish psorosperms” was placed on a firmer basis by Leydig (1851) and Lieberkühn. The former found numerous forms in marine fish, and he discovered in species which live free in the gall bladder of cartilaginous fishes that the psorosperms originated in a manner similar to the gregarines. Lieberkühn (1854) studied the Myxosporidia in the bladder of the pike (fig. 93, _a_, _b_, _d_), and observed their amœboid movements, as well as the formation of the spores, from each of which a small amœboid body escaped, a discovery that was confirmed by Balbiani. The same author also found that spiral filaments were enclosed in the so-called polar body, _i.e._, the polar capsule of the psorosperm spores, and that these could be protruded (fig. 93, _d_, and fig. 95).