The sense-organs of the Starfish are chiefly the discs of the tube-feet. Round the edges of these there is a special aggregation of sense-cells; elsewhere, as in the skin of the back, only isolated sense-cells are found, and it becomes impossible to speak of a sense-organ.
A prolongation of the radial nerve-cord extends outwards along one side of each tube-foot. This is often spoken of as the "pedal nerve," but the term nerve is properly retained for a mere bundle of axons such as we find in the higher animals, whereas the structure referred to contains the bodies of nerve-cells as well as their outgrowths or cell-fibres and is therefore a prolongation of the nerve-cord.
Fig. 192.—Diagrammatic longitudinal section through a young Asteroid passing through the tip of one arm and the middle of the opposite interradius. This diagram is generalised from a section of Asterina gibbosa. ab, Aboral sinus; ax, axial sinus; ax1, basal extension of axial sinus forming the inner perihaemal ring-canal; br, branchia = gill = papula; g.r, genital rachis; mp, madreporite; musc.tr, muscle uniting a pair of ambulacral ossicles; nerv.circ, nerve-ring; n.r, radial nerve-cord; oc, eye-pit; oss, ossicles in skin; p.br, peribranchial sinus; p.c, pore canal; perih (on the right), perihaemal radial canal, (on the left), outer perihaemal ring-canal; py, pyloric caecum; rect, rectum; rect.caec, rectal caeca; sp, spines; st.c, stone-canal; t, median tentacle terminating radial canal; w.v.r, water-vascular radial canal. The genital stolon (not marked by a reference line) is seen as an irregular band accompanying the stone-canal, its upper end projects into a small closed sac, also unmarked, which is the right hydrocoele or madreporic vesicle.
At the base of the terminal tentacle the radial nerve-cord ends in a cushion. This cushion is called the "eye," for it is beset with a large number of cup-shaped pockets of the ectoderm. Each pocket is lined partly by cells containing a bright orange pigment and partly by visual cells each of which ends in a small clear rod projecting into the cavity of the pit (Fig. 193, A, vis.r). The pit is apparently closed by a thin sheet of cuticle secreted by the most superficial cells.
An exposed nervous system and simple sense-organs such as the Starfish possesses lend themselves admirably to the purposes of physiological experiment, and so Starfish have been favourite "corpora vilia" with many physiologists.
Fig. 193.—A, longitudinal section of a single eye-pit of Asterias. s.n, Nucleus of supporting cell; vis.n, nucleus of visual cell; vis.r, visual rod. B, view of the terminal tentacle showing the eye-pits scattered over it. (After Pfeffer.)
The light-perceiving function of the eye is easily demonstrated. If a number of Starfish be put into a dark tank which is illuminated only by a narrow beam of light they will be found after an interval to have collected in the space reached by the beam of light.[[445]] If all the median tentacles but one be removed this will still be the case; if, however, they are all removed the Starfish will exhibit indifference to the light.
If the under surface of a Starfish be irritated by an electric shock or a hot needle, or a drop of acid, the tube-feet of the affected area will be strongly retracted, and this irritation will be carried by the pedal nerves to the radial nerve-cord, with the result that finally all the tube-feet in the groove will be retracted and the groove closed by the action of the transverse muscle connecting each ambulacral ossicle with its fellow. If, on the other hand, the back of a Starfish be irritated this may produce a contraction of the tube-feet if the irritation be strong, but this will be followed by active alternate expansions and contractions, in a word, by endeavours to move. Preyer[[446]] by suspending a Starfish ventral surface upward, by means of a small zinc plate to which a string was attached which passed through a hole bored in the back and through the mouth, caused movements of this description which lasted for hours. Irritation of the back causes also activity of the local pedicellariae, which open their valves widely and then close them with a snap in the endeavour to seize the aggressor.