| Placozoa - the 'plate' animals |
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| This
phylum's 'place'
relative to other phyla:
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Selected
taxa within this taxon:
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1 (2?) species described!
Trichoplax adhaerens
Discovered in the late 1880's living in a
laboratory aquarium.
General Characteristics
Extremely simple animals with the smallest
amount of DNA ever measured for any type of animal. Body consists
of only a few
thousand cells.
Amorphous, flat (<3 mm) with two distinct sides. A tissue layer composed of two tissue layers:
Life History
Reproduction
Asexual - binary fission or budding
Hypotheses on the Phylogenetic Position of Placozoa (no fossil record of placozoans.):
Some molecular (RNA) evidence places
in the diploblastic assemblage along with Poriferans, Cnidarians and Ctenophorans,
though other
evidence suggests Placozoan may be neotonous planula-type larva of a bilateria
ancestor. LESSON: Beware of simplicity
when constructing phylogenetic relationships.
Placozoan Diversity
Of only two species described, one
is in doubt because it has never been seen since its description in 1896
(recorded only from Gulf of Naples) . Placozoans are so cryptic that
their diversity might be greater.
Lecture Sources:
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