Presumptive photoreceptor structures of the trochophore ofHarmothoë imbricata(Polychaeta)

Abstract

The behaviour of the Harmothoë trochophore changes with age, the larva being phototropic initially and later photonegative. The trochophore possesses two ocelli midway between the prototroch and the apex in a mid‐lateral position. They appear first at the eighth day of development and grow to be kidney‐shaped structures. There is a pigment cup derived from a single cell that encloses a rhabdomeric type of photoreceptor apparatus that is also derived from a single (or rarely two) cell. In the late trochophore (14 days old) an organ of different origin and formation but of presumed photoreceptor type begins to develop among nerve cell bodies below the apex of the animal. This structure consists of an array of membranes developed from both cilia and microvilli. The cilia are of 9 + 2 configuration.