[1] Traditionally, dicyemids have been grouped with the Orthonectida in the phylum Mesozoa and, from 2017, molecular evidence[2][3] appears to confirm this.

[5][6] The phylum (or class if retained within Mesozoa) contains three families, Conocyemidae, Dicyemidae and Kantharellidae,[7] which have sometimes been further grouped into orders.

The anterior region of the organism is termed a calotte and functions to attach the parasite to folds on the surface of its host's renal appendages.

[5] This extreme segregation of habitats allows multiple species of dicyemids to comfortably exist within the same host while not still competing for space or resources (by occupying different ecological niches).

In a study done on octopuses, it was found that Dicyemida that had similarly shaped calottes rarely coexisted in the same individual host, which suggested a strong level of competition for habitat.

Some scientists have speculated that when closely related species of dicyemids coexist in the same region, such as in the case of D. misakiense and D. japonicum, competition for habitat causes them to evolve to develop two distinct calotte shapes.

It has long been assumed that this sexually produced infusoriform, which is released when the host eliminates urine from the kidneys, is both the dispersal and the infectious stage.

[10] Some part of the dicyemid life cycle may be tied to temperate benthic environments, where they occur in greatest abundance[citation needed].

Dicyemids have never been reported from truly oceanic cephalopods, who instead host a parasitic ciliate fauna[citation needed].