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MESOZOA. By Stacy Slavinski. Biology of the Invertebrates p. 169. Phylum Mesozoa.

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By Stacy Slavinski

Biology of the Invertebrates p. 169

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Phylum Mesozoa

  • From the Greek Mesos for middle and zoon an animal. Mesozoa is a “Middle Animal”. It is called this because it is believed to be between unicellular protists and the triploblastic flatworms in their level of organization. slides/phyla/mesozoavs.gif

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General Characteristics

  • Very small animals, ranging from 0.5 mm - 7 mm.

  • -Bilaterally symmetrical

  • -No organs or tissues

    -No nervous system, respiratory, circulatory, or digestive system.

  • -Elongate body with a ciliated epidermis

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More General Characteristics

  • Body contains no internal cavity

  • Body is only two cell layers

  • Two-tissue layer triploblast

  • Has some cells develop inside other cells

  • Reproduction is quite complex involving both sexual and asexual aspects

  • All are endoparasites on other marine invertebrates

  • Less than 50 cells makeuptheir body.

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General Knowledge

  • They are poorly understood animals and a small phylum.

  • Know fossil mesozoans are known, and little research has been conducted on them.

  • There are about 50 known species and they are divided into two classes that are not related to each other at all.



  • The classes are separated by looking at their asexual parasitic phases.

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  • Parasites of several marine invertebrates including:

    -Platyhelminthes, Echinodermata, Mollusca and Annelida.

  • Locomotion is through ciliary gliding, although the body is also capable of flexion.

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  • During sexual stage they are gonochorisitc (male and female)

  • - they have no central tube-cell at this phase, but the space within the layer of ciliated cells is filled with eggs and sperm.

  • - males release their sperm into the sea, the sperms enter body of any females encountered, and fertilize her eggs.

  • - fertilized eggs grow into ciliated larva (with only a few cells).

  • - fertilization occurs outside the body.

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Larva Stage of Orthonectida

  • Larva leaves the mothers body and enters body of suitable host.

  • -The larva metamorphoses into a plasmodium that causes damage to its host, notably through suppression of sexual organs.

  • -Inside the host, it loses its cilia and grows larger to form a plasmodium (similar to multicellular amoeba).

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More information about the Larva Stage:

-Plasmodium has many nuclei and is called multinucleate.

-Bits of the plasmodium break off and form new plasmodia.

-Eventually this gives rise to the sexual, it leaves the host, and the life cycle is complete.

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This gives an idea of how the Orthonectida forms into the adult form. Although, the mesozoa is a poorly studied parasite.


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Another Look at the Orthonectida life cycle.

Biology of the Invertebrates p. 170

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Rhombozoans mesozoa/meso002b

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  • Also called Dicyemida, are parasites of cephalopods (Octopus and Squid).

  • This parasites lives in the kidneys of its host.

  • This class has more of a complicated life cycle, which is not completely understood.

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Rhombozoans Continued

  • The axial cell is made up of smaller cells called axoblasts.

    -The axoblasts give rise to either vermiform, which is long and thin, asexual larvae called nematogens, or sexually reproducing individuals called rhombogens.

    -The two forms are physically identical, except that in nematogen stage the axoblasts produce more nematogens and in the rhombogen stage they produce infusorigens, which serve as the animals gonads.

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Rhombozoans Continued

  • The eggs are fertilized inside the axial cell where they develop into infusoriform larvae.

    -The larvae quickly develop adult number of cells.

    -Each species has a definite number of cells in its adult form.

    -Infusoriform larvae then leaves the axial cell and the hosts body, through the hosts urine.

    -They then sink to the sea floor, where they grow by cell enlargement instead of cell addition.

    -How the larvae reenters its host and becomes nematogens is not really known.

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This is the life cycle, showing both the adult nematogen and the adult rhombogen in a cephalopod host.

Dicycema life cycle

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This also provides us with an insight into how the Dicyemida forms into the adult. However, as mentioned before, not much is known about these parasites.

Dicyemida (order of classRhombozoa)

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Another look at the life cycle of the Rhombozoa

Biology of the Invertebrates p. 170

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Who are the mesozoa’s ancestors?

  • Some speculate that the origin of Mesozoa is either degenerate turbellarians or as primitive multicellular animals related to ciliated protist.

  • Since they animals are so poorly studied and understood, researchers have tried to come up with many possible ideas of the mesozoa’s ancestors.

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Salinella, is the hypothetical ancestor. Some believe that this indicates, to a small degree where mesozoa in fact came from.

One Possible Ancestor MESOZOA/DIAGBW/MESO001B

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Article on Origin of Mesozoa

  • An article titled Origin of the Mesozoa inferred from 18S rRNA Gene Sequence.

  • The authors: Jan Pawlowski, Juan-Ignacio Montoya-Burgos, Jose Fahrni, Jean Wuest, and Louisette Zaninetti indicate, after looking at the 18S rRNA sequence that the Mesozoa branch early in animal evolution, closely to nematodes and myxozoans.

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Article results:

  • Their results indicate a separate origin of rhombozoids and orthonectids.

  • With this new information, they believe even placing the two in the same phylum may need to be reevaluated.

  • The article is quite fascinating, however, to go into details would take more than time permits. I suggest, if interested in learning more about the mesozoa, to read this article. Other articles I found were about the same gene sequence, and how this contributes to their origin. As I stated several times, the knowledge about mesozoa is poorly studied/understood.

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  • Web Sites and Article Used:

    - slides/phyla/mesozoavs.gif

    - mesozoa/meso002b

    -Pawlowski J, MontoyaBurgos JI, Fahrni JF, et al. Origin of the Mesozoa inferred from 18S rRNA gene sequences MOL BIOL EVOL 13 (8): 1128-1132 OCT 1996

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 Thank You For Your Time


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