Nerve cells evidently of a sensory nature have been found in Eledone cirrhosa on the inside of the mantle in a limited area near the stellate ganglion. The cells, whose number on each side of the body is no less than 50, are in close association with a special thin layer of muscles, and obviously must have a proprioceptive function. The whole complex can therefore be regarded as a muscle receptor which from its situation may be termed the substellar organ.

The muscular component of this organ consists of fibres arranged in very flat bundles which, anastomosing with one another, form a plexiform layer situated under the stellate ganglion and the stellar nerves. It may be called substellar muscle plexus (ss-plexus for short). The area occupied by it, which is roughly semicircular in shape, extends to the points where the stellar nerves penetrate the muscles or a little beyond these points (Text-fig. 1). The ss-plexus, although situated close to thecompact muscle of the mantle, does not appear to have anatomical relation with the latter; it has, however, direct connexions with strands of muscle fibres reaching theplexus from two directions. The fibres coming from the medianside belong to the muscle attaching the mantle to the visceral sac in which runs the pallial nerve (or mantle connective). This muscle, called lateral pallial adductor (see Tippmar, 1913) istwisted in such a way that its bundles coming from the anterior region of the visceral sac insert into the mantle behind the stellate ganglion, and those originating posteriorly insert in front of it.