1) Perhaps the simplest free-living metazoan; an evolutionarily highly significant phylum initially thought to contain only 1 marine species: Trichoplax adhaerans
2) Superficially resembles a large ameba (2-3 mm diam; 25 µm thick):
flattened body has three layers; composed of only 4 cell types:
cover cells: flattened cells with single flagellum (monociliated) form the upper surface
cylinder cells: columnar cells with single flagellum (monociliated) surrounded by a ring of microvilli form the lower surface
gland cells: non-flagellated secratory cells interspersed with columnar cells on the lower surface
mesenchyme cells: a watery, syncytial (multinucleate) middle layer that resembles the syncytia of hexactinellid sponges
upper & lower surfaces formed by single layers of cells that are not true epithelia because no basal lamina is present
3) Move like ameba: no defined anterior end, but well defined dorsal and ventral surfaces (will quickly right itself if inverted)
4) Feeds by 'humping up' over algae & detritus and secreting digestive enzymes into the cavity; digestion is extracellular; absorption occurs via microvilli and ventral surface of cylinder cells
5) Reproduce clonally in three ways: fission (split into two equivalent halves), fragmentation (a small fragment crawls away), or budding (flagellated buds swim away). Sexual reproduction is not well understood (eggs develop in ventral cells, but sperm are not known)
6) Trichoplax appears to be a very early metazoan lineage intermediate between sponges and the Eumetazoa
- outer layers are more epithelium-like than pinacoderm but are still not true epithelia because a basal lamina is lacking
- adults clearly motile, but lack consistent anterior or posterior end