Invertebrate zoologists consider the phyla Dicyemida
, Orthonectida, Placozoa, Cycliophora, Entoprocta (Kamptozoa), Chaetognatha, Pterobranchia, and Enteropneusta along with the order Seisonidae.
Owing to their simple body plan, orthonectids were grouped together with Dicyemida
, microscopic parasites found in ceph-alopods, in a clade called Mesozoa, a sister group to Metazoa.
A classic example is the question of monophyly of the Mesozoa, specifically with respect to the parasitic phyla Orthonectida and Dicyemida
. This question is aggravated by a well-known and yet still unsolved problem of long branch attraction.
All members of the phylum Dicyemida
are found in the renal sacs of benthic cephalopod molluscs (Nouvel, 1947; McConnaughey, 1951; Hochberg, 1990).