The ultrastructure of a gutless annelid, Parenterodrilus gen. no v. taenioides (= Astomus taenioides) (Polychaeta, Protodrilidae)

An ultrastructural investigation of the mouthless protodrilid Parenterodrilus taenioides shows that the residual gut, with its very narrow ciliated lumen, has lost any digestive function. Salivary glands are retained in some anterior segments and their canals converge in a ventral area of the prostomium and open at the epidermal surface. The epidermal ciliation is much more developed than in any other protodrilid species. In the middle and posterior body segments the enlarged epidermal surface area is covered by a cuticle bearing an array of epidermal microvilli which are probably the site of active transport of dissolved organic compounds. The presence of numerous subcuticular coated vesicles and lysyosomes in the support cells suggests that particulate material from the ambient milieu is taken up by endocytosis and finally digested in the epidermis. The origin and nature of the organic substances that constitute the food of P. taenioides are still unknown. There are no symbiotic bacteria in the cuticle, epidermis, or residual gut. Abundant intracellular crystals in the nonfunctional gut indicate that this residual organ plays a role in the bioaccumulation of environmental substances which enter the body through the epidermis. The epidermis is the only absorptive surface of this unique polychaete, which is devoid of both a functional gut and symbiotic bacteria.