ABSTRACT
Silver staining methods have been applied to the nervous system of Velella, Two histologically distinct plexuses are described under the headings ‘open’ and ‘closed’ systems. The open system consists of neurones with fine processes which run for distances of up to 2 mm, retaining their independence ins pite of frequent contacts with other fibres. The fibres of the closed system are large and run together, forming a nerve-net in which neurofibrillar material from different neurones intermingles; it is provisionally to be regarded as a syncytium. A certain type of ‘fibre’ in this system is believed to arise secondarily by the drawing out of adhesion connexions into long strands. Free nerve-endings resembling growing-points occur in both systems. The two systems occur throughout the ectoderm, but in the invaginated ectoderm the open system is poorly developed. The functions of the two systems are not known, but the closed system is probably specialized for through-conduction.
Neuro-sensory cells occur in the external ectoderm, making contact with fibres of both open and closed systems. No specialized endings have been found in a muscular region examined. No nerve-rings or centres have been found. Nerves are sparsely distributed in the endoderm, but they lie independently of one another and of ectodermal nerve-fibres crossing the mesogloea between the invaginated and external ectoderm layers.
