ULTRASTRUCTURE OF PRESUMED OCELLI IN PARENTERODRILUS-TAENIOIDES (POLYCHAETA, PROTODRILIDAE) AND THEIR PHYLOGENETIC SIGNIFICANCE

Abstract

Three different types of presumed unpigmented ocelli have been found in the anterior end of Parenterodrilus taenioides, a small gutless interstitial polychaete. The type-1 ocelli are located in the palps and four ocelli have been found along the length of each palp. The type-2 and type-3 ocelli lie close together in the head segment and are located in posterior ganglionic expansions of the brain. There is one pair of the minute type-2 ocelli but at least two pairs of the type-3 organs, which are the largest ocelli. In each ocellus the sensory cells are of the ciliary type and possess two cilia whose plasma membranes branch into numerous microvilli. With the exception of the type-1 ocelli they consist of a sensory cell and a supportive cell. In each ocellus the supportive cell forms a thin cup-shaped envelope around the densely packed ciliary branches. The type-1 ocelli consist of a single cell forming an intracellular vacuole (phaosome) which contains less densely packed microvillus-like structures. In particular, the structure of these ocelli is compared with that in other polychaetes, with special emphasis on the remaining genera of the Protodrilida.