A marine species which represents the simplest animal phylum, the Placozoa. In culture these animals are a few millimeters in diameter and about 10 μm thick. There are four cell types: (a) the flat covering cells of the dorsal epithelium, each with a single cilium, (b) ventral cylindrical, ciliated epithelial cells, (c) ventral, non-ciliated gland cells that secrete digestive enzymes, and (d) cells containing contractile fibers in their cytoplasm which reside in the space between the dorsal and ventral epithelia. The ventral ciliated cells are adhesive and capable of resorbing materials that have been digested from the substrate. Trichoplax has the smallest genome of all animals surveyed to date, 40 mb, and its haploid chromosome number is 6. A Pax gene has been isolated and shown to be expressed in the margin of Trichoplax. The gene product may specify the differentiation of the fiber cells that are regarded as protomesodermal cells. See Pax genes.
Subjects: Genetics and Genomics.