The phylogenetic position of dicyemid mesozoans
offers insights into spiralian evolution.
Syncytia, while uncommon among vertebrates, do occur in a variety of animals including flatworms, mesozoans
, and insects (Barnes, 1980).
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.
Although recent studies have revealed that they might not be truly primitive animals deserving the name of mesozoans (Katayama et al., 1995; Kobayashi et al., 1999), they are still one of the most interesting groups of lower invertebrates.
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. Zool, Sci.
The development of the vermiform embryos of two mesozoans, Dicyema acuticephalum and Dicyema japonicum.
The highly stereotypic cleavage pattern referred to as "spiral cleavage" occurs in most of the extant invertebrate phyla, including the molluscs, annelids, vestimentiferans, pogonophorans, echiurans, sipunculids, nemerteans, gnathostomulids, mesozoans
, and polyclad turbellarians.
The dicyemid mesozoans have long been the subject of a phylogenetic controversy (Brusca and Brusca, 1990; Willmer, 1990).
We determined almost the entire sequence of 18S rDNA in two species of dicyemid mesozoans and three species of turbellarians (Platyhelminthes).
This study does not support a close relationship between Trichoplax and mesozoans.
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).