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From the Greek Mesos for middle and zoon an animal. Mesozoa is a

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3. 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

4. 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 makeup their body.

5. 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. -Orthonectida -Rhombozoans The classes are separated by looking at their asexual parasitic phases.

6. Orthonectida

7. Orthonectida Parasites of several marine invertebrates including: -Platyhelminthes, Echinodermata, Mollusca and Annelida. Locomotion is through ciliary gliding, although the body is also capable of flexion.

9. Larva Stage of Orthonectida

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

12. Another Look at the Orthonectida life cycle.

13. Rhombozoans

14. Rhombozoans 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.

15. 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.

16. 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.

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

18. Dicyemida (order of class Rhombozoa) 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.

19. Another look at the life cycle of the Rhombozoa

20. 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.

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

22. 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.

23. 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.

24. References 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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