The Ultrastructure of the Sensory Dorsal Organ of Crustacea1

Abstract The present study compares the ultrastructure of "dorsal organs" on the anterior, dorsal carapace of the syncarid Anaspides tasmaniae and the crangonid shrimp Crangon crangon. Although the species are not closely related phylogenetically, and the elements of their dorsal organs are arranged differently, they are remarkably similar ultrastructurally. The common elements include an island of thinner epicuticle lying across the aperture of a hole through the surrounding cuticle, squarish in the case of Anaspides, and lozenge shaped in Crangon. This whole area appears to be flexible. At the centre of the thin region is a tabular invagination of cuticle ending blindly in Anaspides and with a pore at the bottom in Crangon. The tube is surrounded by a single large cell with extensive internal membranes and basal vacuoles or vesicles. This part of the organ is not innervated. Four small papillae are disposed about this central region, in quincunx formation in Anaspides, and a pair each side in a row in Crangon. The cuticle thins further over the papillae and the underside is closely associated with four sensory dendrites so that each organ is innervated by a total of sixteen neurons. The four dendrites beneath each papilla have basal bodies and cilliary microtubules typical of mechanosensors. The region close to the tips of the dendrites is surrounded by non-cellular material and the dendrites are separated from each other by a series of sheath cells. On the basis of this similarity, and because the relationship between these elements, as evidenced by studies of the external structure across a wide range of taxa, is strongly conserved, we propose that the organs described here belong to a particular class of "dorsal organs" which we call sensory dorsal organs (of Laverack). On the basis of the ultrastructure, and the conservation of the proximity of