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.
Among animals, there were Mesozoa
, Gnathostomulida, Bryozoa, Cycliophora, Myzostomida, and Nemertea; also there are Rotifera, Nematomorpha, Scalidophora, Acanthocephala, Entoprocta, and Gastrotricha from Coelomata.
By this means it was revealed that all the flagella and cilia in animals including sponges, Mesozoa
and choanoflagellates showed the twoheaded outer arm profile.
But few people have seen representatives of many phyla such as the strange, parasitic Mesozoa
, or the Placozoa, the most primitive and simple of all animals.
ANIMALIA 1,320,000 MESOZOA
METAZOA 1,320,000 Porifera (sponges) 10,000 Cnidaria (hydras, jellyfish, corals, etc) 10,000 Platyhelminthes (flatworms) 20,000 Nematoda (roundworms) 25,000 Echinodermata (sea uchins, etc.) 6,000 Choradata (fish, birds, mammals, etc.) 45,000 Arthrododa (crabs, spiders, insects, etc) 1,085,000 Mollusca (snails, squids, etc) 12,000 Annelida (segmented worms) 12,000
, the Placozoa and various groups of Protozoa"; (11) M.
2 and 5 Other Metazoa Phyla Other Eukaryote Phyla Chaetognatha Ciliophora Ctenophora Other Alvelata Entoprocta Amoebozoa Hemichordata Cryptophyta Loricifera Loukozoa Gnathifera Glaucophyta Onochophora Choanozoa Porifera Picozoa Rotifera Foraminifera Xenoturbeiidea Rhodophyta Acanthocephala Myzozoa Chordata Apusozoa Cycliophora Heliozoa Gnathostomulida Percolozoa Kinohycha Euglenozoa Mesozoa
Haptophyta Nematomorpha Metamunada Placozoa Cercozoa Priapulida Radiozoa Sipuncula Uncultured unicellular Uncultured Metazoa Eukaryote Acknowledgments