The morphology of the spines of the ophiuroid Ophiocomina nigra is described, with particular reference to the nervous system and the sensory and secretory structures of the epidermis. The nervous system is composed of two main spinal nerves, located at the centre of the spine, and their associated branches. There are three secretory cell-types described: (1) fibrillar secretory cells which produce long, javelin-shaped secretory pack-ages and, occurring exclusively in the basal two thirds of the spine, penetrate deeply with their basal regions lying close to the axial nerve running through the spine centre; (2) granular secretory cells, which also penetrate deep within the spine, contain secretory granules in the form of spherical dense vesicles (~1.3 μm in diameter); and (3) goblet secretory cells, filled with packages of loose amorphous material, are superficial in location and usually found associated with a type A ciliated sensory cell. The secretions of the fibrillar and granular secretory cells are thought to perform the functions of defence and feeding respectively. A number of different ciliated sensory cell-types have been identified. Apart from the situation with the goblet cells, no close association was found between secretory and sensory cells. It is suggested that the nervous, sensory and secretory cells act together to form a mucous secretion system with centralized, rather than local control. This system appears to operate when it is advantageous to produce secretion all over the body simultaneously once any portion is stimulated. Stimulation of sensory cells might result in axonal excitation of the spinal nerves and hence to the entire epineural nervous system.