520 ECHINODERMATA ECHINOIDEA chap. 



muscles are stretched the tone is lowered, and this loss of tone 

 extends to the neuron controlling the muscle, and vice versa. 

 When the spines on being gently stimulated bend towards 

 the point of stimulation, this is due to the contraction of the 

 muscles on the side towards the point of stimulus, for if the 

 superficial plexus of nerve -fibres be cut through so that the 

 stimulus has to pursue a round-about course the spine will bend 

 towards the direction from which the stimulus comes. The 

 bending of the spines away from the stronger stimulus is like- 

 wise due to the muscles on the side towards the stimulus. It is 

 caused by a sudden fall of tone in these muscles, which causes 

 them to yield to the tone of the muscles on the opposite side, 

 and this fall of tone is due to a fall of tone in the neurons, for 

 it can be produced by chemicals, and the direct action of all 

 chemicals applied to muscle is to raise tone. 



In Arbacia this form of reaction cannot be produced; the 

 spines respond to stimuli of all degrees of intensity by converg- 

 ence towards the point of stimulation. 



When a general skin-irritant like dilute acetic acid, or even 

 strong light, is applied to the skin of a Sea-urchin the spines 

 bend alternately to all points of the compass, or, in a word, rotate. 

 This is due to the fact that the weight of the inclined spine 

 stretches the muscles of one side and so renders them more open 

 to the general stimulus ; these muscles in consequence contract, 

 and so move the spine to a new position in which other muscles 

 are stretched, and a similar result follows. A continuation of 

 this process brings about rotation. 



When a piece of glass rod or other light object is laid on the 

 spines of a Sea-urchin it naturally, by its weight, presses asunder 

 the spines and stretches their muscles on one side, thus lowering 

 the tone. If now the skin be stimulated at any point the piece 

 of rod will be rolled by the spines towards the point of stimula- 

 tion. This is caused by the fact that the muscles of the spines 

 holding the rod are made more receptive by being stretched, and 

 therefore they contract more than do the others in response to 

 the stimulation, and so the rod is rolled onwards on to the next 

 spines, which then act in the same manner. This passage of 

 stimulus is entirely independent of direct nervous connexion 

 between the bases of the spines, for it will traverse at right 

 angles a crack going clean through the shell ; it is merely the 



