The SPX-organ in Boreomysis arctica (Krøyer) (Crustacea Malacostraca Mysidacea) was investigated light and electron microscopically. The organ consists of a group of cells (the SPX-cells) and a vesicle surrounded by a connective tissue sheath. It is situated near the base of the sensory papilla of the eye-stalk. Neurosecretory material is produced in the SPX-cells and transported in axon-like projections from these cells into the vesicle. These processes contain no neurotubuli. Numerous fibres from an afferent nerve emanating from the medulla terminalis also enter the vesicle, where they form a dense irregular meshwork. This nerve transports no neurosecretory material. There are numerous synaptic contacts between the afferent nerve fibres and the neurosecretory processes from the SPX-cells. The neurosecretory material released from them accumulates in haemocoelic spaces in the vesicle. Release is most probably effected by the afferent nerve. © 1969 Springer-Verlag Berlin Heidelberg.
