Studies on the Myoneural Physiology of Echinodermata. Iv. the Lantern Retractor Muscle of Parechinus: Responses to Stimulation by Light.

The jaw apparatus of the regular echinoids is moved by a complex of muscles. Of these the largest and most accessible are the lantern retractor muscles which originate on the auricles and insert upon the pyramids which enclose the teeth. Preliminary experiments using Parechinus angulosus Mortensen showed that, while in this species these muscles could not be stimulated indirectly by way of electrical shocks applied to the radial nerves, they are photosensitive, contracting on exposure to light after a period of dark-adaptation. The existence of photosensitive nerves in echinoids is already well known from the analysis by Millott (1954) of the shadow response of Diadema. The account which follows is limited to a qualitative description of the results of photostimulation of the lantern retractor muscle, a necessary background before more detailed quantitative studies can be made. Parechinus angulosus was collected between tide marks at Port Alfred in the eastern part of the Cape Province of South Africa. The animals are usually found in crevices beneath large stones by day, but wander freely in the rock pools at night. In laboratory aquaria they show a marked tendency to avoid bright light. After collection the animals were brought back to the laboratory where they were kept in porcelain sinks. Provided care is taken to remove all faeces and the water is violently aerated to produce a constant movement, the animals remain in good condition for a fortnight to 3 weeks.