Selenidium

All species in this genus infect marine invertebrates.

These species infect the gut of polychaete worms.

The trophozoites are vermiform with an apical complex. They have few epicytic folds. A dense array of microtubules lies under a trilayered pellicle.

Syzygy occurs in the tail-to-tail, head-to-head and lateral positions.

The gamonts are extracellular. They are foliaceous or cylindroid in shape and have longitudinal striations.

The oocysts are spherical or ovoid, are 12-18 microns in diameter and their wall is 1 micron thick. They have four infective sporozoites each.

The sporozoites undergo schizogony (merogony).