(only have two germ layers: ectoderm and endoderm) also lack a coelom, those technically do not have an acoelomate body plan because they lack the mesoderm germ layer. In acoels, the mouth opens directly into a large endodermal syncytium, while in nemertodermatids and xenoturbellids, there is a sack-like gut lined by unciliated cells.

A defining feature is a digestive system lacking nerve cells.