Anatomical and ultrastructural study of the pharyngeal bulb in Protodrilus (Polychaeta, Archiannelida). I. Muscles and myo-epithelial junctions.

The pharyngeal bulb of Protodrilus is both a muscular and an epithelial organ whose function is the drawing up of food particles. The muscular system of the bulb is formed of tightly connected antagonistic muscles: the bulbus muscle and the sagittal and "grating plate" muscles. All of them are composed of obliquely striated fibers whose ultrastructural characteristics are similar to those of Hirudina and even more to those of epitokous forms of Nereidae and Syllinadae. Myo-epithelial cells do not exist in the pharyngeal bulb of Protodrilus contrary to what was previously thought; the muscles and the stomodeal epithelium are united by junction areas on both sides of the basal lamina. These myo-epithelial junctions may be compared of the myoepidermic junctions known in several Arthropods. A comparison of the ultrastructural features of the bulbus muscle fibers of Protodrillus (Protodrilidae) and Trilobodrilus (Dinophilidae) shows that the Protodrilus fiber clearly belongs to the obliquely striated type classically found in Polychaeta, while the Trilobodrilus fiber is a very peculiar type of obliquely striated fiber. These differences do not agree with Jägerstens hypothesis on the unity of the Archiannelida established on the basis of a structural similarity of the bulbus muscles.