The Phylum Mesozoa

Etymology:- From the Greek Mesos for middle and zoon an animal.

Characteristics of Mesozoa:-
1)Bilaterally symmetrical.
2)Has no organs or tissues.
3)Body contains no internal cavity.
4)Body possesses no digestive tract (gut).
5)Body only two cell layers in most places.
6)Has no nervous system.
7)Has some cells develop inside other cells.
8)Reproduction quite complex involving both sexual and asexual aspects.
9)All are endoparasites on other marine invertebrates.

The Mesozoa are a small phylum of small and poorly understood animals. They have very simple bodies, often consisting of less than 50 cells. All known species are internal parasites of marine invertebrates.


Scientists are unsure whether they represent a still existing remnant from the early stages of the development of multicellular animals, or a degenerate form descended from the Platyhelminthes. No fossil mesozoans are known and very little research been done on them, so consequently we know very little about them.The 50 or so known species are divided into two classes which some experts believe are not actually related at all. Apart from differences in life cycles the two classes are easily separated by looking at their respective asexual parasitic phases (forms). In the Rhombozoa this stage is long thin and ciliated, in the Orthonectida it is shaped more like an amoeba and it isn't ciliated. Mesozoans have no gaseous exchange organs, no circulatory system, no nervous system and no digestive system.

The Orthonectida

The Orthonectida are parasites on a wide range of marine invertebrates including Platyhelminthes, Echinodermata, Mollusca and Annelida. During the sexual stage they are gonochoristic (male and female). During this phase they have no central tube-cell, instead the space within the layer of ciliated cells is filled with eggs or sperm. The males release their sperm into the sea. the sperms enter the body of any females they find and fertilise her eggs. These fertilised eggs grow into a ciliated larva (consisting of only a few cells). This larva now leaves the mothers body and enters the body of a suitable host. Inside the host it loses its cilia and grows larger to form a plasmodium (something a bit like a multicellular amoeba). This plasmodium has many nuclei and is called multinucleate. Bits of this plasmodium break off and form new plasmodia. Eventually the plasmodia give rise to the sexual which soon leave the host and the life cycle is complete.


Rhombozoans, sometimes called Dicyemida in some older texts are parasites of cephalopods (Octopus and Squid), they live in the animals kidneys. Rhombozoans have a more complicated life cycle. Their basic body plan is a long thin central cell, called an axial or tube cell, surrounded by a coat of smaller ciliated cells which are arranged spirally around the axial cell. Some authors equate this with a two cell-layer body plan.

The axial cell contains smaller cells called axoblasts. These axoblasts give rise to either, vermiform(long and thin) asexual larvae called nematogens sexually reproducing individuals called rhombogens. the two forms are physically identical as far as we know. The only difference being that in the nematogen stage the axoblasts produce more nematogens and in the rhombogen stage they produce infusorigens which serve as the animals gonads (organs which produce eggs and sperm). The eggs are fertilised inside the axial cell where they develop into infusoriform larvae which quickly develop the adult number of cells. Each species has a definite number of cells in its adult form. This infusoriform larvae then leaves the axial cell and the hosts body, with its urine. They then sink to the sea floor where they grow by means of cell enlargement rather than by cell addition. It is not currently know how these larvae re-enter their hosts and become nematogens.




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