Phylum Mesozoa

7/31/2019 Phylum Mesozoa

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MESOZOA

Biology of the Invertebrates p. 169 By Stacy Slavinski


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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.

Phylum Mesozoa

www.ldeo.columbia.edu/.../life/ 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 marineinvertebrates

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 onthem.

There are about 50 known species andthey are divided into two classes that arenot related to each other at all.

-Orthonectida-Rhombozoans

The classes are separated by looking attheir asexual parasitic phases.


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Orthonectida

biodidac.bio.uottawa.ca/.../ZOO/MESOZOA/DIAGBW/MESO003B.GIF


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Orthonectida

Parasites of several marineinvertebrates including:

-Platyhelminthes, Echinodermata,

Mollusca and Annelida.

Locomotion is through ciliary gliding,

although the body is also capable offlexion.


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OrthonectidaDuring 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 ofsuitable 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 growslarger to form a plasmodium (similar to

multicellular amoeba).


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More information about the Larva Stage:

-Plasmodium has many nuclei and is calledmultinucleate.

-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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Orthonectida

This gives an idea ofhow the Orthonectida

forms into the adultform. Although, themesozoa is a poorlystudied parasite.

http://www.biologie.hu-berlin.de/~zoologie/sammlung/Tafeln/Mesozoa.html


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Another Look at theOrthonectida life cycle.

Biology of the Invertebrates p. 170


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Rhombozoans

tolweb.org/tree/eukaryotes/animals/ mesozoa/meso002b


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Rhombozoans

Also called Dicyemida, are parasites ofcephalopods (Octopus and Squid).

This parasites lives in the kidneys of itshost.

This class has more of a complicatedlife cycle, which is not completelyunderstood.


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RhombozoansContinued The axial cell is made up of smaller cells called

axoblasts.-The axoblasts give rise to eithervermiform, which is long and thin, asexual

larvae called nematogens, or sexuallyreproducing individuals called rhombogens.

-The two forms are physically identical,except that in nematogen stage the

axoblasts produce more nematogens and inthe rhombogen stage they produceinfusorigens, 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 numberof cells.

-Each species has a definite number of

cells in its adult form.-Infusoriform larvae then leaves the axialcell and the hosts body, through the hostsurine.

-They then sink to the sea floor, wherethey grow by cell enlargement instead ofcell addition.

-How the larvae reenters its host and

becomes nematogens is not really known.


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Dicycema life cycle This is the life cycle,

showing both theadult nematogenand the adult

rhombogen in acephalopod host.

www.ldeo.columbia.edu/dees/ees/life/slides/phyla/dicyema


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Dicyemida (order of classRhombozoa)

This also providesus with an insightinto how the

Dicyemida formsinto the adult.However, asmentioned

before, not muchis known aboutthese parasites.

http://www.biologie.hu-berlin.de/~zoologie/sammlung/Tafeln/Mesozoa.html

A h l k h lif l f


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Another look at the life cycle ofthe Rhombozoa

Biology of the Invertebrates p. 170


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Who are the mesozoas

ancestors? Some speculate that the origin of Mesozoa

is either degenerate turbellarians or as

primitive multicellular animals related tociliated protist.

Since they animals are so poorly studied

and understood, researchers have tried tocome up with many possible ideas of themesozoas ancestors.


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One Possible Ancestor

Salinella, is the

hypothetical

ancestor. Some

believe that this

indicates, to a smalldegree where

mesozoa in fact

came from.

biodidac.bio.uottawa.ca/.../ZOO/ MESOZOA/DIAGBW/MESO001B


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Article on Origin of Mesozoa

An article titled Origin of the Mesozoainferred from 18S rRNA Gene Sequence.

The authors: Jan Pawlowski, Juan-Ignacio Montoya-Burgos, Jose Fahrni,Jean Wuest, and Louisette Zaninettiindicate, after looking at the 18S rRNAsequence that the Mesozoa branchearly in animal evolution, closely tonematodes 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 tobe reevaluated.

The article is quite fascinating, however, to go intodetails would take more than time permits. Isuggest, if interested in learning more about the

mesozoa, to read this article. Other articles I foundwere about the same gene sequence, and how thiscontributes to their origin. As I stated severaltimes, the knowledge about mesozoa is poorly

studied/understood.


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References Web Sites and Article Used:

-www.teachingbiomed.ac.uk/bsl1999/bs146/biodiversity/mesozoa.html-www.earthlife.net/inverts/mesozoa.html

-http://science.kennesaw.edu/~jelirnber/Invertzoo/LecMesozoa/Mesozoa

-www.ldeo.columbia.edu/life/ slides/phyla/mesozoavs.gif

-www.biodidac.bio.uottawa.ca/ZOO/MESOZOA/DIAGBW/MESO003B.

-http://www.biologie.huberlin.de/~zoologie/sammlung/Tafeln/Mesozoa

-www.tolweb.org/tree/eukaryotes/animals/ mesozoa/meso002b

-www.ldeo.columbia.edu/dees/ees/life/slides/phyla/dicyema

-www.biodidac.bio.uottawa.ca/ZOO/MESOZOA/DIAGBW/MESO00B

-www.biologie.huberlin.de/zoologie/sammlung/Tafeln/Mesozoa.html

-Pawlowski J, MontoyaBurgos JI, Fahrni JF, et al.Origin of the Mesozoa inferred from 18S rRNA genesequences MOL BIOL EVOL 13 (8): 1128-1132 OCT 1996
http://www.teachingbiomed.ac.uk/bsl1999/bs146/biodiversity/mesozoa.htmlhttp://www.earthlife.net/inverts/mesozoa.htmlhttp://www.earthlife.net/inverts/mesozoa.htmlhttp://science.kennesaw.edu/~jelirnber/Invertzoo/LecMesozoa/Mesozoahttp://www.biologie.huberlin.de/~zoologie/sammlung/Tafeln/Mesozoahttp://www.biodidac.bio.uottawa.ca/ZOO/MESOZOA/DIAGBW/MESO00Bhttp://www.biodidac.bio.uottawa.ca/ZOO/MESOZOA/DIAGBW/MESO00Bhttp://www.biodidac.bio.uottawa.ca/ZOO/MESOZOA/DIAGBW/MESO00Bhttp://www.biodidac.bio.uottawa.ca/ZOO/MESOZOA/DIAGBW/MESO00Bhttp://www.biologie.huberlin.de/~zoologie/sammlung/Tafeln/Mesozoahttp://science.kennesaw.edu/~jelirnber/Invertzoo/LecMesozoa/Mesozoahttp://www.earthlife.net/inverts/mesozoa.htmlhttp://www.earthlife.net/inverts/mesozoa.htmlhttp://www.earthlife.net/inverts/mesozoa.htmlhttp://www.teachingbiomed.ac.uk/bsl1999/bs146/biodiversity/mesozoa.html


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Thank You For Your Time

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Phylum Mesozoa