XVI ASTERIAS — PHYSIOLOGY OF NERVOUS SYSTEM 447 



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. 



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 sub- 



