The uninjured Starfish in moving pursues a definite direction, one arm being generally directed forwards, but this may be any one of the five. The tube-feet of this arm are directed forwards when they are stretched out, by the slightly unequal contraction of the longitudinal muscles of opposite sides of the foot, which persists even when the circular muscles of the ampulla are contracting. They thus may be said to swing parallel to the long axis of the arm. The tube-feet of the other arms assist in the movement, and hence swing obliquely with reference to the long axis of the arm to which they belong, although they move parallel to the general direction in which the Starfish is moving. A change in the direction of the swing of the tube-feet will bring about a change in the direction of the movement of the animal as a whole. If now the connexion of each radial nerve-cord with the nerve-ring be cut through, each arm will act as a separate Starfish and will move its tube-feet without reference to the movement of those in the other arms, so that the animal is pulled first one way and then another according as the influence first of one arm and then of another predominates. Similarly, when a Starfish is placed on its back, it rights itself by the combined action of the tube-feet of all the arms, extending them all as widely as possible, those which first catch hold being used as the pivot for the turning movement. If, however, the radial nerve-cords are cut through, each arm tries to right itself and it is only by chance that the efforts of one so predominate as to turn the whole animal over. From these experiments it is clear that the nerve-ring acts as co-ordinator of the movements of the Starfish, that is to say as its brain.
If a section be taken across the arm of a Starfish (Fig. 191), it will be seen that between the V-shaped ridge constituting the radial nerve-cord and the radial water-vascular canal there are two canals lying side by side and separated from one another by a vertical septum. These canals are not mere splits in the substance of the body-wall, but have a well-defined wall of flattened cells. They are termed, for reasons which will be explained subsequently, perihaemal canals, and they open into a circular canal called the "outer perihaemal ring," situated just beneath the water-vascular ring-canal (Fig. 192, perih). These canals originate as outgrowths from the coelom. From their upper walls are developed the muscles which connect the pairs of ambulacral ossicles and close the groove, and also those which connect each ossicle with its successor and predecessor and help to elevate or depress the tip of the arm.
In most of the higher animals the processes of many of the ganglion-cells are connected together in bundles called "motor nerves," which can be traced into contact with the muscles, and thus the path along which the stimulus travels in order to evoke movement can clearly be seen. No such well-defined nerves can be made out in the case of the Starfish, and it is therefore interesting when exceptionally the paths along which stimuli travel to the muscles can be traced. This can be done in the case of the muscles mentioned above. Whereas they originate from the dorsal walls of the perihaemal canals, ganglion-cells develop from the ventral walls of these canals, which are in close contact with the nerve-cord, so that the nervous system of the Starfish is partly ectodermic and partly coelomic in origin. Stimuli reaching the ectodermic ganglion-cells are transmitted by them to the nervous part of the wall of the perihaemal canal and from that to the muscular portion of the same layer of cells.
Besides the radial perihaemal canals and their connecting outer perihaemal ring there are several other tubular extensions of the coelom found in the body-wall. These are:—
(1) The "inner perihaemal canal," a circular canal in close contact with the inner side of the outer perihaemal canal (Fig. 192, ax1).
(2) The "axial sinus" (ax) a wide vertical canal embedded in the body-wall outside the stone-canal. This canal opens into the inner perihaemal canal below; above it opens into several of the pore-canals and into the stone-canal. The separation of the axial sinus from the rest of the coelom is the remains of a feebly marked metamerism in the larva.
(3) The "madreporic vesicle," a closed sac embedded in the dorsal body-wall just under the madreporite. This sac by its history in the larva appears to be a rudimentary counterpart of the water-vascular system, since this organ in correspondence with the general bilateral symmetry of the larva is at first paired. Into this a special process of the genital stolon projects.
(4) The "aboral sinus" (Fig. 192, ab), a tube embedded in the dorsal body-wall running horizontally round the disc. The aboral sinus surrounds the genital rachis (see p. [452]) and gives off into each arm two branches, the ends of which swell so as to surround the genital organs. It has no connexion with the axial sinus though the contrary has often been stated by Ludwig.[[447]]
(5) The "peribranchial spaces," circular spaces which surround the basal parts of the papulae (Fig. 192, p.br).
Besides these, large irregular spaces have been described as existing in the body-wall by Hamann[[448]] and other authors, but for various reasons and especially because they possess no definite wall they appear to be nothing more than rents caused by the escape of CO2 gas during the process of decalcifying, to which the tissues of the Starfish must be subjected before it is easy to cut sections of them.