Acoela

Acoela are a small phylum of simple, flatworm‑like marine animals. They comprise about 140 described species and are typically only a few millimeters long. They are mostly free‑living in shallow coastal waters and are noted for their very simple body plan compared with other flatworms.

Anatomy and biology: Acoela are dorsoventrally flattened and lack a true body cavity (acoelomates). The outer body surface is a syncytial epidermis, usually bearing cilia. Their digestive system is simple and not a through gut; digestion is largely intracellular and there is no anus. They lack specialized circulatory and respiratory organs, with gas exchange occurring by diffusion. The nervous system is relatively simple, often described as a diffuse nerve net with an anterior concentration of nerve cells in some species, and some have light‑sensitive eyespots. Many features of their development and organ systems have made them a focus in studies of early bilaterian evolution.

Reproduction and development: Most Acoela are hermaphroditic. Reproduction occurs sexually in many species, and several can reproduce asexually by fragmentation or budding. Developmental modes vary among taxa, with some species exhibiting direct development from eggs.

Classification and phylogeny: Historically classified within Platyhelminthes, Acoela are now considered part of Xenacoelomorpha, together with Nemertodermatida. The precise placement of Xenacoelomorpha within Bilateria is debated, with analyses placing them as sister to Nephrozoa (the group containing protostomes and deuterostomes) or as sister to all other bilaterians. Ongoing molecular and morphological work continues to refine their position.

Ecology and significance: Acoela inhabit a range of shallow‑water habitats worldwide, often feeding on small invertebrates and detritus. Their primitive features provide valuable insights into early bilaterian evolution, including the origins of the gut, body cavity, and mesoderm.