Evolutionary significance of fine structure of archiannelid eyes

Summary The structure of the ocelli of representatives of four of the five families of archiannelids (Polychaeta: Annelida) was studied by light and electron microscopy. The apparent photoreceptoral organelle in each species is an array of microvilli (rhabdomere). Cilia were observed in the eyes of only a couple of specimens in one species of archiannelid (Nerilla antennata). They were unassociated with the rhabdomeres; we regard them as adventitious. Support is given by this study to the theory that the photoreceptoral organelle of the ancestral annelid was a rhabdomere. Other features of the ocelli are described and illustrated.We acknowledge the support of a grant-in-aid of research (GM 10292) from the United States Public Health Service; the skillful transformation of our sketches into finished drawings by Phyllis Thompson; the assistance of Jean Brandenburger in the electron microscopy of villar particles; and the critical reading of the paper by Mrs. Brandenburger and Drs. Colin O. Hermans and Ralph I. Smith