acoelomorph

Acoelomorphs are a group of simple, soft-bodied animals that belong to the phylum Acoelomorpha. They are primarily marine organisms, though some species inhabit freshwater environments. The name "acoelomorph" derives from the Greek words *akoelos*, meaning "without a cavity," and *morphē*, meaning "form," referring to their lack of a body cavity (acoelomate condition).

Acoelomorphs are characterized by their dorsoventrally flattened bodies, lacking a true coelom, anus, or specialized circulatory and respiratory systems. Their digestive system is typically a simple sac-like structure with a single opening that functions as both mouth and anus. Some species exhibit complex cellular structures, including a syncytial epidermis and a diffuse nervous system with nerve cords and ganglia.

The phylum includes two main classes: Acoela and Nemertodermatida. Acoela, the more diverse group, contains species like *Convoluta roscoffensis*, which exhibits a unique symbiotic relationship with algae. Nemertodermatida, a smaller class, includes organisms such as *Nemertoderma westbladi*, notable for its highly reduced body plan and genetic similarities to bilaterians.

Acoelomorphs were traditionally classified as bilaterians due to their bilateral symmetry, but recent molecular phylogenetic studies suggest they may represent an early-branching lineage distinct from other bilaterians. Their genetic and developmental traits, such as the presence of Hox genes and a triploblastic body plan, have sparked debate among researchers regarding their evolutionary position.

These animals play ecological roles as predators or scavengers, feeding on small invertebrates, detritus, or symbiotic algae. Their simple yet intriguing biology continues to provide insights into the evolution of complex animal body plans.