The following points highlight the twelve main features of Phylum Placozoa.
1. Phylum Placozoa are minute, shapeless, multicellular mobile flattend animals.
2. 2-3 mm in diameter.
3. Body of Phylum Placozoa is composed of two outer layers with monociliated epithelial cells.
4. Epithelial cells lack a basal lamina.
5. An inner layer containing loose contractile columnar cells with a single flagellum in each cell between the two layers (loose mesenchyme-like cells).
6. Between the two layers of epithelial cells there is a fluid-filled space which contains contractile fibrous cells.
7. Body cavity, digestive system and nervous system absent.
8. Phylum Placozoa have no organs or tissues.
9. No mouth and no sign of phagocytosis.
10. Digestion seems extracellular type in Phylum Placozoa.
11. Asexual reproduction by budding, fragmentation or binary fission.
12. They also reproduce sexually and each individual produces single oocyte.
Example:
Trichoplax adherens (Fig. 11.18), the only representative which was discovered in marine aquarium in Europe. Later on the same species was discovered in the various parts of the marine environments. It is the simplest and most primitive metazoan.
