Studies using transcriptomic data of a dicyemid, Dicyema japonicum, have also suggested that orthonectids are protostomes (Lu et al, 2017); and another analysis on mitochondrial genomes and nuclear genes suggested that orthonectids are members of the phylum Annelida (Schiffer et al, 2018).
The phylogenetic position of dicyemid mesozoans offers insights into spiralian evolution.
Although adult morphology varies vastly across the group, many core Lophotrochozoa (including the Mollusca, Annelida [including Vestimentifera, Pogonophora, and Echiura, and possibly Myzostoma, and Sipunculida], Gnathostomulida, Nemertea, dicyemid
Mesozoa, Entoprocta, and some Platyhelminthes [including polyclad Turbellaria, Catenulida, and Macrostomida]) exhibit spiral cleavage, a highly conserved pattern of early development.
Patterns of cell division and cell lineages of the vermiform embryos of dicyemid mesozoans were studied in four species belonging to four genera: Conocyema polymorpha, Dicyema apalachiensis, Microcyema vespa, and Pseudicyema nakaoi.
Detailed comparative studies on cell lineages and organization of infusoriform embryos are also indispensable in separating dicyemid taxa.
Fourteen new species of dicyemid mesozoans from six Japanese cephalopods, with comments on host specificity.
The development of the hermaphroditic gonad in four species of dicyemid mesozoans.
The dicyemid mesozoans have long been the subject of a phylogenetic controversy (Brusca and Brusca, 1990; Willmer, 1990).
Phylogenetic analyses using nucleotide sequences of 5S rRNA suggested that a dicyemid (Dicyema misakiense) diverged early among such lower metazoa as sponges, cnidarians, and flatworms (Ohama et al., 1984; Hori and Osawa, 1987).
We determined almost the entire sequence of 18S rDNA in two species of dicyemid mesozoans and three species of turbellarians (Platyhelminthes).
Phylum 1 Rhombozoa (Rhombozoans, Dicyemids
Sexually mature individuals with cell numbers below this threshold have so far been reported only from secondarily simplified, commensal, or parasitic species with very simple bodyplans, such as the mesozoan dicyemids
, which live in the kidneys of benthic cephalopods and consist of only 10 to 40 cells (4).