Another interesting feature is the refractile body located beneath the massive flagella bundle characterizing the large crescents. It is not related morphologically to the ciliate blepharoplast (a "9+2" centriole-related structure found in cryptogams such as cycads, "Ginkgo biloba" and algae e.g. "Euglena" and "Chlamydomonas"). This flagella-associated structure observed in the large selenomonad can perhaps best be described as a "basal sac" formed by an invagination (in-folding) of the "polar organelle" region of the bacterial cell membrane in the middle of the concave side of the organism so that it lies directly behind the flagella. It appears to be unique in prokaryotes so far examined since in other bacteria possessing polar organelles, the structure is situated beside and around the flagella insertion bases in the cell membrane, but never lying behind them in the cytoplasm as in the case of the large selenomonad.