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Biology of the Invertebrates p. 169   By Stacy Slavinski
                 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.

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
•   Less than 50 cells makeup their body.
         General Knowledge
• They are poorly understood animals and a
    small phylum.
•   Know fossil mesozoans are known, and
    little research has been conducted on
•   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
•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

-Bits of the plasmodium break off and form new

-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
• This class has more of a complicated
  life cycle, which is not completely
 Rhombozoans Continued
• The axial cell is made up of smaller cells called
     -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
 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
     -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.
      Dicycema life cycle
                                                           • This is the life cycle,
                                                             showing both the
                                                             adult nematogen
                                                             and the adult
                                                             rhombogen in a
                                                             cephalopod host.
         Dicyemida (order of class
                                                                          • This also provides
                                                                            us with an insight
                                                                            into how the
                                                                            Dicyemida forms
                                                                            into the adult.
                                                                            However, as
                                                                            before, not much
                                                                            is known about
                                                                            these parasites.
              Another look at the life cycle of
              the Rhombozoa

Biology of the Invertebrates p. 170
Who are the mesozoa’s
• 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.
       One Possible Ancestor
                                                           • Salinella, is the
                                                             ancestor. Some
                                                             believe that this
                                                             indicates, to a small
                                                             degree where
                                                             mesozoa in fact
                                                             came from. 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
• 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

       The END

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