630-600 million years ago


Higher flatworms possessed a centrally located mouth but evolved a mucociliary mechanism in the pharynx (which higher vertebrates retain in their respiratory system), coordinated muscle contractions called peristalsis, and microvilli.

In planarians, the mouth is not located in the head but is located more posteriorly or even centrally, suggesting a link to more primitive, radially symmetric animals. In flatworm orders Macrostomida and Notandropora digestion is extracellular and a stable epithelial gut lining exists (Beklemishev 2, p. 192). Since they lack a circulatory system, flatworm cells must be located near the gut to obtain food by diffusion, just as in coelenterates. The gut (which is pigmented in the above image) is highly branched as a result and serves a distributive (vascular) function in addition to a digestive function.

In humans, the respiratory tract is lined by ciliated cells which can sweep mucus away from the lungs to the mouth, where it can be swallowed. This mucociliary mechanism was originally a feature of the digestive system, rather than the respiratory system. All animals with a digestive cavity, ranging from simple coelenterates through vertebrates possess cilia move particles which are caught in mucus toward the digestive cavity (Fretter). Although this is a feeding mechanism in lower animals (especially filter feeders), higher vertebrates utilize this same mechanism to remove microbes and debris from the respiratory tract and take it to the stomach to be destroyed. Early bilateran animals (such as flatworms), evolved a pharynx which was ciliated, and possessed longitudinal and circular muscle layers (Dougherty, p. 197, Beklemishev 2, p. 196). Microvilli increase the surface area of the intestine as in higher animals (Hickman) and many unicellular glands are present (Beklemishev 2, p. 196).