1 / 25

MESOZOA - PowerPoint PPT Presentation

  • Updated On :
  • Presentation posted in: General

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

I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.

Download Presentation


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.

- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -

Presentation Transcript


By Stacy Slavinski

Biology of the Invertebrates p. 169

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

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

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.

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.



  • Parasites of several marine invertebrates including:

    -Platyhelminthes, Echinodermata, Mollusca and Annelida.

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


  • 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.

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).

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.

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


Another Look at the Orthonectida life cycle.

Biology of the Invertebrates p. 170

Rhombozoans mesozoa/meso002b


  • 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.

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.

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.

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

Dicycema life cycle

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)

Another look at the life cycle of the Rhombozoa

Biology of the Invertebrates p. 170

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.

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

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.

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.


  • 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

 Thank You For Your Time


  • Login