Some aspects of the neurobiology of Ophiuroids : with special reference to Ophiura texturata (L.) (Echinodermata, Ophiuroidea)

Abstract

A study has been made of the anatomy and physiology of the nervous system of Ophiura texturata, involving the use of light and electron microscopy and of extracellular and intracellular electrophysiological recording techniques. A previously unknown ciliary feeding structure in O.texturata has also been described. The nerve cords of O. texturata contain a system of large neurones that are an order of magnitude larger than the nerve cells occurring in members of other echinoderm classes. These large neurones have been designated as "giant" neurones. The size of the giant neurones has made it possible to trace their extent within the nerve cords and thus to produce the first detailed description of the cellular structure of an echinoderm nervous system. The radial nerve cords consist of a chain of structurally similar segments, and degeneration studies have shown that each segment contains a separate population of neurones. The circumoral nerve ring which has previously been regarded as the controlling centre of the echinoderm nervous system, has a structure which is consistent only with providing a functional connection between adjacent radii and not with the task of central integration. The electrical activity of single units within the ectoneural part of the nerve cords of O.texturata has been recorded in response to a variety of stimuli, by the use of extracellular suction electrodes. Decrementally conducted compound potentials, reported in previous electrophysiological studies of echinoderm nervous systems, were not recorded. Intracellular recordings of the activity within single giant cells have also been obtained. This is the first report of intracellularly recorded nervous activity in any echinoderm. The intracellular work is of a preliminary nature and suggestions for further study are made. The genital shields in O.texturata are covered by an array of ciliated ridges and non-ciliated grooves. An examination of this structure indicates that it is specialized for a form of ciliary/mucus suspension feeding. These structures also provide a specialized preparation for the study of some aspects of the function of the sub-epidermal nerves in echinoderms.