The phylum Placozoa (Gr. plax,
, tablet, plate,+ zoon
was proposed in 1971 by K. G. Grell
to contain a single species, Trichoplax
(Figure 12-3A), a tiny (2 to
3 mm) marine form that had been
considered either a mesozoan or a
cnidarian larva by various workers.
The body is platelike and has no symmetry,
no organs, and no muscular or
nervous system. It is composed of a
dorsal epithelium of cover cells and
shiny spheres, a thick ventral epithelium
containing monociliated cells
(cylinder cells) and nonciliated gland
cells, and a space between the epithelia
containing fluid and fibrous cells
(Figure 12-3B). The organisms glide
over their food, secrete digestive
enzymes on it, and then absorb the
products. Grell considers Trichoplax
diploblastic, with dorsal
epithelium representing ectoderm and
ventral epithelium representing endoderm
because of its nutritive function.
The phylogenetic position of placozoans
is uncertain, although recent
molecular evidence places them as a
sister group to the phylum Cnidaria.
A, Trichoplax adhaerens is a marine, platelike animal only 2 to 3 mm in diameter. The only member of
phylum Placozoa, it has the most primitive features of any known metazoan. B, Section through
adhaerens, showing histological structure.