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This phylum's 'place'
relative to other phyla:
- Metazoan
- Triploblast
- ‘Platyhelminthes-Rotifer-Lophotrochozoa’
group
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Selected taxa within this taxon:
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85 species
Body Plan
Extremely simple!
- Few
dozen cells forming a two-tissue layer triploblast.
- Elongate body with a ciliated
epidermis.
- No nervous, respiratory,
circulatory, or digestive system
Natural
History
- All endoparasitic in marine
invertebrates.
One class
mostly invades gonads of marine invertebrates and when in host
forms a multinucleated plasmodium body form.
Members of the other class are
parasites of cephalopod mollusks and have a complex poorly
understood Life
cycle (in one stage, individuals develop completely to an
adult stage within the body of the previous generation).
Phylogeny
Would be considered as diploblastic
but:
- Some researchers suggest that inner
layer is derived from mesoderm (endoderm tissue lost).
Tissue is non-digestive and produces gametes. This
would be consistant with considering mesozoans as degenerate
turbellarian flatworms
- Some characteristics are so unique (for example,
embryos develop within other cells) that mesozoan are
considered by some to be in their own subkingdom apart from
their Subkingdoms Radiata, Myxozoa, and Bilateria (and even an
independent origin from other multicellular animals).
Why doesn’t simplicity always imply
that a group is an an early (primitive) evolutionary branch? (
i.e. what selective pressures lead to more simple body plans?)
Within the Mesozoa, there is still
controversy on whether the two major groups are monophyletic,
with some recent molecular evidence supporting common ancestry,
though more recent analyses suggesting that the Mesozoa should
be divided into two new phyla.
Lecture Sources:
- Pechenik.
Jan A. 2000. Biology of the
Invertebrates. McGraw-Hill, New York.
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