ORGAN OF BELLONCI AND SINUS GLAND IN 3 DECAPODS FROM ATLANTIC HYDROTHERMAL VENTS - RIMICARIS-EXOCULATA, CHOROCARIS-CHACEI, AND SEGONZACIA-MESATLANTICA

Crustaceans of the hydrothermal vent field of the Mid-Atlantic Ridge p resent graded variations of eye structure. In the shrimps Rimicaris ex oculata and Chorocaris chacei, external eyestalks are missing or reduc ed and conventional compound eyes are lacking. The crab Segonzacia mes atlantica has small eyestalks. Our objective was to localize and descr ibe the organ of Bellonci and the sinus gland usually included in the eyestalks. In Rimicaris exoculata and Chorocaris chacei, the organs of Bellonci are well developed and contain onion body-like structures. T hey are located in front of the cerebral ganglia close to the hemielli psoid bodies, a position unusual among decapods. Each organ of Bellonc i extends to a complex cuticular structure which is probably a sensory pore. In the two shrimps, the optic lobes are located within the brai n. The four optic neuropile layers are present. The sinus gland is sma ll and cupula-shaped, with usual histological features. It is located between the interna and externa medullae, adjacent to a hemolymph lacu na. In Segonzacia mesatlantica, the eyestalks contain massive nervous masses with numerous and complex chiasmata from which only the lamina ganglionaris is slightly separated. The organ of Bellonci and the sinu s gland are both located inside the eyestalks on opposite sides. The o rgan of Bellonci contains onion bodies but no distinguishable sensory pore. The sinus gland is large with visible patches of neurosecretory secretions and is surrounded by a thick layer of connective tissue.