ABSTRACT
Placozoa is an early-diverging phylum of motile seawater invertebrates with arguably the simplest animal body plan. They are flat, ameboid-like animals that lack body symmetry and are composed of a limited number of cell types that are arranged along ciliated dorsal and ventral epithelia, sandwiching a network of mesenchyme-like cells (Grell & Benwitz, 1971; Grell, 1972; Schulze, 1883, 1892). Placozoans lack a dorsal/ventral body axis, so the commonly used terms dorsal and ventral epithelia should be considered purely descriptive, and alternatively referred to as upper and lower epithelia, respectively.
