Species of Monocystis are typically endoparasites of earthworms and occur in their coelom and seminal vesicles.
Nuclei in Monocystis is single and vesicular with spherical or ellipsoidal form it contains usually one nucleolus or karyosome which stain deeply with iron haematoxylin. Nuclear membrane is delicate and bears pores. Nucleoplasm contains four chromosomes (haploid number).