The connections of the sensory organ of Bellonci with the brain in Isopoda (crustacea).

The peduncle linking the organ of Bellonci with the brain was examined in Sphaeroma serratum and Anilocra frontalis. This peduncle, in its extension to the brain, becomes a nerve-like tract with bundles of pedicles originating from the sensory cell bodies located in the organ of Bellonci. It ends at the level of the medulla interna in an alveolar region resulting from the swelling of the sensory pedicle terminations. At this level three types of connections have been observed. The first is characterized by afferent synapses to the brain with, in the sensory pedicle endings, structures similar to the presynaptic ribbons noted by some authors in photoreceptors of arthropods. The two other types include nerve fibres originating from the brain, one with small electron lucent vesicles, a second displaying larger vesicles with a core of medium density. These fibres form efferent synapses to the organ of Bellonci. The sensory differentiation of the organ of Bellonci in Isopoda is confirmed but its true role is not specified.