Mesozoa

The Mesozoa are enigmatic, minuscule, worm-like parasites. It is still unclear as to whether they are degenerate platyhelminthes (flatworms) or truly-primitive, basal metazoans. Generally, these tiny, elusive creatures consist of a somatoderm (outer layer) of ciliated cells surrounding one or more reproductive cells. Decades ago, Mesozoa was classified as a phylum. But molecular phylogeny studies have shown that the mysterious mesozoans are polyphyletic. That is, they consist of two unrelated groups.

As a result of these recent findings in molecular biology, the label mesozoan is now often applied informally, rather than as a formal taxon. Some workers previously classified Mesozoa as the sole phylum of the lonely subkingdom Agnotozoa.

Life cycle
Mesozoa have complex life cycles alternating between asexual and sexual generations. In the nephridia of a cephalopod are numerous wormlike organisms (nematogens) that are composed of a fixed number of somatic cells. Within the center of the nematogens are axial cells. These cells give rise to new nematogens. The nematogens turn into rhombogens to prepare for sexual development. Sexual development begins in the axial cell of the rhombogen in an area called the infusorigen. Here, the amoeboid sperm develops. The jacket cells turn into eggs. Sperm emerges from the axial cell and fuses with the egg forming an infusoriform larva. The larva then crawls out. Eventually, the octopus is infected again by the young vermiform (worm-like) stage.

Evolution
Mesozoa were once thought to be evolutionary intermediate forms between Protozoans and Metazoans, but now they are thought to be degenerate or simplified metazoa. Their ciliated larva are similar to the miracidium of trematodes, and their internal multiplication is similar to what happens in the sprocysts of trematodes. Mesozoan DNA has a low GC-content (40%). This amount is similar to ciliates, but ciliates tend to be binucleate. Others relate mesozoa to a group including annelids, planarians, and nemerteans.

Groupings
The two main mesozoan groups are the Rhombozoa and the Orthonectida. Other groups sometimes included in the Mesozoa are the Placozoa and the Monoblastozoa.

Monoblastozoans consist of a single description written in the 19th century of a species that has not been seen since. As such, many workers doubt that they are a real group. As described, the animal had only a single layer of tissue.

Rhombozoan mesozoans
Rhombozoa, or dicyemid mesozoans, are found in the nephridia (kidneys) of cephalopods (squid and octopuses). They range from a few millimeters long with twenty to thirty cells that include anterior attachment cells and a long central reproductive cell called an axial cell. This axial cell may develop asexually into vermiform juveniles or it may produce eggs and sperm that self-fertilize to produce a ciliated infusiform larva.

There are three genera: dicyema, pseudicyema, and dicyemennea.

Orthonectid mesozoans
Orthonectida are found in the body spaces of various marine invertebrates including tissue spaces, gonads, genitorespiratory bursae. This pathogen causes host castration of different species.

The best known of Orthonectida is the parasite of brittle stars. The multinucleate syncytial stage lives within tissues and spaces of the gonad but can spread into arms. It causes the destruction of starfish ovary and eggs to cause castration (the male gonads are usually unaffected). The stages of the plasmodium develop into more plasmodia by simple fragmentation; at some point, they decide to go sexual. The syncytia are monoecious (either male or female), but young syncytia can fuse to produce both male and female. The males are ciliated and smaller than the females. The females and the males leave the starfish and mate in the sea. Tailed sperm enters the female and fertilizes the numerous oocytes. Each oocyst produces a small ciliated larva which makes way to another star.