The structure of the human microsporidium found by Yachnis and colleagues in two AIDS patients (Am. J. Clin. Pathol. 106: 535-43, 1996) (hereafter referred to as HMY) was investigated by light and transmission electron microscopy and compared with Thelohania apodemi Doby, Jeannes et Raoult, 1963, a microsporidian of small rodents. The fine structure of the HMY was found to be similar to that of Trachipleistophora hominis Hollister, Canning, Weidner, Field, Kench et Marriott, 1996. Characteristic is the presence of a thick layer of electron dense material on the outer lace of the meront plasmalemma, which is maintained during the whole life cycle and which later persists as an electron dense coat on the sporophorous vesicle (SPOV). However, HMY is distinguished from T. hominis during sporogony, as two types of SPOV and spores are formed in HMY. One type of SPOV contains thick-wallcd spores (usually 8 or more in number) with anisofilar polar filaments of 7 + 2 pattem, while the other type contains only two thin-walled spores with a smaller number (3-5) of isofilar polar filament coils. The HMY differs from T. apodemi which also forms SPOV with 8 spores inside, but the spores of which are larger in size and have 9 + 2 polar filament pattern.
Development of a new myxozoan parasite Tetracapsula bryozoides gen. n. et sp. n. in the coelomic cavities of Cris-latella mucedo Cuvier is described. Uninucleate proliferative cells are formed within well-defined sacs, the wall of which is one cell thick. The sacs, of different sizes according to age, are free floating and are conspicuously moved about within the coelomic fluid by the ciliary movements of the host. Division of the proliferative cells produces spherical cells of different sizes with nuclei of commensurate size. The largest cells enter sporogony by dividing into ten cells. Four of these become capsulogenic cells arranged as an anterior group, each giving rise to a spherical polar capsule containing a polar filament, possibly without prior formation of an external tube or, at most, very transient formation of these. Four valvogenic cells enclose the two sporoplasms and overlie the capsulogenic cells except at the points of exit of the polar filaments from the polar capsules. The two uninucleate sporoplasms are packed with endoplasmic reticulum, numerous mitochondria with tubular cristae and sporoplasmosomes which are distributed peripherally. Both sporoplasms produce secondary cells. Typical myxosporean features of the wall cells of the sac and all stages within the sac are: nuclei with granular nucleoplasm and prominent nucleolus, gap junctions between cells consisting of thickened membranes with cross connections, and haplosporosomes. A new genus is established for the parasite, defined as having development limited to uninucleate pseudoplasmodia within a sac of parasite origin, each uninucleate sporogonie stage giving rise to one spore with tetraradial symmetry, composed of four shell valves, four anterior polar capsules and two uninucleate sporoplasms with secondary cells. No plasmodia are formed. The genus is placed within the order Multivalvuli-da, in a new family Saccosporidae, defined as having development within a sac of parasite origin and sporogony without external tube or microtubules during polar capsule formation.