Fine structure of the ocellus of Sarsia tubulosa (Hydrozoa, Anthomedusae)

Abstract

The fine structure of the ocellus of Sarsia tubulosa is described. The ocellar cup is formed of pigment cells and receptor cells. The receptor cells outnumber the pigment cells in almost a 2:1 ratio. Lateral extensions of neighbouring pigment cells enclose a distal region of 2 to 10 receptor cells. The receptor cell body is 5–7 μm in diameter with an apical extension (20–60 μm long) that reaches the ocellar cavity. A cilium (9+2 microtubules) arises from the distal part of the receptor cell. The ciliary membrane forms lateral microvilli. The tips of a number of cilia are swollen into large vesicles forming a “cornea”. The central region of the ocellar cavity contains extracellular electron dense homogeneous material surrounded by swollen ciliary tips and small vesicles. The close apposition between the plasma membrane covering the distal part of adjacent receptor cells as well as the adjacent ciliary shafts suggests the presence of gap junctions. The basal part of each receptor cell forms an axon. The axons of receptor cells form 3 to 4 nerve bundles that join to form the optic nerve. Synapses occur between receptor cell bodies, between axons and receptor cell bodies and among axons.