Ultrastructure of the attachment cells of the organ of Bellonci in Gammarus setosus (Crustacea, Amphipoda).

Abstract

The structure of the organ of Bellonci indicates that it is a sensory organ able to receive a variety of external stimuli. The suspension of the organ was investigated in the marine amphipod Gammarus setosus by light, scanning, and transmission electron microscopy. The organ is suspended ventrally by a cord whose specialized cuticular attachment is in the roof of a canal under the interantennal angle of the head. The conical end of the cord consists of five epithelial attachment cells sheathed by processes of 21 cells arranged in five tiers inside a basal lamina. The attachment cells end within a knob in the centre of a diaphragm. The cuticular attachment is confined to the knob and consists of endocuticular rods within pits on the surfaces of the attachment cells. Dorsally, the long processes of the attachment cells form a net over the surface of the organ and reunite in the dorsal suspensor, which anchors the organ in the epidermis at the crown of the head. The attachment cells are characterized by extensive hemidesmosomes and dense core granules at both attachment sites and by microtubules that extend through their entire length of up to 1 mm in the adult. The large size, cephalic position, elaborate structure, and suspension of the organ suggest that it is of considerable importance in the sensory capability of aquatic Malacostraca.