DEVELOPMENT OF A MYXOZOAN PARASITE TETRACAPSULA-BRYOZOIDES GEN-N ET SP-N IN CRISTATELLA-MUCEDO (BRYOZOA, PHYLACTOLAEMATA)

Development of a new myxozoan parasite Tetracapsula hryozoides gen. n. et sp. n. in the coelomic cavities of Cristatella mucedo Cuvier is de scribed. Uninucleate proliferative cells are formed within well-define d sacs, the wall of which is one cell thick. The sacs, of different si zes according to age, are free floating and are conspicuously moved ab out within the coelomic fluid by the ciliary movements of the host. Di vision of the proliferative cells produces spherical cells of differen t sizes with nuclei of commensurate size. The largest cells enter spor ogony by dividing into ten cells. Four of these become capsulogenic ce lls arranged as an anterior group, each giving rise to a spherical pol ar capsule containing a polar filament, possibly without prior formati on of an external tube or, at most, very transient formation of these. Four valvogenic cells enclose the two sporoplasms and overlie the cap sulogenic cells except at the points of exit of the polar filaments fr om 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 cel ls of the sac and all stages within the sac are: nuclei with granular nucleoplasm and prominent nucleolus, gap junctions between cells consi sting of thickened membranes with cross connections, and haplosporosom es. A new genus is established for the parasite, defined as having dev elopment limited to uninucleate pseudoplasmodia within a sac of parasi te origin, each uninucleate sporogonic stage giving rise to one spore with tetraradial symmetry, composed of four shell valves, four anterio r polar capsules and two uninucleate sporoplasms with secondary cells. No plasmodia are formed. The genus is placed within the order Multiva lvulida, 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.