630-600 million years ago


acoel digestive tract


In the earliest worms, the mouth was located in the center of the body, the digestive system consisted of a gastrovascular cavity with only one connection to the environment, and cells lining this cavity performed phagocytosis of microbes.

Flatworms of the order Acoela lack a pharynx, an intestine, and an anus. The digestive tract has only one opening, as in cnidarians. The mouth is not located at the anterior end of the animal, but rather more centrally. Although acoels are bilateral animals with a head, the organization of their digestive system with a single, more centrally located opening is more similar to that of cnidarians than to other bilaterans (Hickman). In acoels, food is digested by phagocytosis by the gut lining instead of by enzymatic breakdown. It is unknown whether this is a reduction from the primitive state of extracellular digestive enzymes found in cnidarians or whether Acoela retain the most primitive type of animal digestion (Fretter, Dougherty).