Phylum Placozoa
Phylum Placozoa
The phylum Placozoa (Gr. plax, plakos, tablet, plate,+ zoon, animal) was proposed in 1971 by K. G. Grell to contain a single species, Trichoplax adhaerens (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.
The phylum Placozoa (Gr. plax, plakos, tablet, plate,+ zoon, animal) was proposed in 1971 by K. G. Grell to contain a single species, Trichoplax adhaerens (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.
 |
| Figure 12-3 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 Trichoplax adhaerens, showing histological structure. |
Support our developers
