Developmental patterns of the hermaphroditic gonad in dicyemid mesozoans (Phylum Dicyemida)

Abstract. The development of the hermaphroditic gonad, the infusorigen, was studied in fixed specimens of 16 species belonging to four genera ( Conocyema, Dicyema, Dicyemennea , and Pseudicyema ) of dicyemid mesozoans. In addition, the literature on the cell lineages of infusorigens of eight other species was reviewed. After an agamete (axoblast) undergoes an unequal first division, excluding a small cell (the paranucleus), the resulting large cell divides to form the infusorigen. Five infursorigen cell lineage patterns can be identified: type I, both the first oogonium and the first spermatogonium are produced at the third division; type II, the first spermatogonium is produced at the second division and the first oogonium is produced at the third division; type III, the first spermatogonium is produced at the second division and the first oogonium is produced at the fourth division; type IV, the first oogonium is produced at the second division and the first spermatogonium is produced at the third division; and type V, the first oogonium is produced at the second division and the first spermatogonium is produced at the fourth division. Later development is similar in members of all genera. Mature infusorigens are similarly organized in all species examined. Sizes of agametes and infusorigen axial cells, the number of infusorigens, and the type of reproductive strategy were not correlated with infusorigen developmental patterns or cell lineage patterns.