xvni PHYSIOLOGY OF NERVOUS SYSTEM 52 I 



result of the mechanical weight of the object and of the juxta- 

 position of the spines. 



If the stimulation be too violent the first spines affected 

 diverge wildly and strike their neighbours with vehemence, so 

 arousing into activity the block musculature of these. This 

 causes them to stand rigidly up, and so the path of the stimulus 

 is barred. 



Now the escape movements of the animal under strong 

 stimulation which Eomanes 1 alludes to are just an example of 

 this handing on of stimulation from spine to spine, not by 

 nervous connexion but by mechanical touch only ; the object in 

 this case is the substratum on which the animal lies, which is, 

 so to speak, rolled towards the point of stimulation, or putting it 

 otherwise, the animal is rolled away from it. Eighting when 

 upset is another example of the same phenomenon ; the aboral 

 spines are stretched by the weight of the animal, and the animal 

 acts as if it were stimulated in the region of the periproct. 

 When a Sea-urchin is in its normal position and is stimulated 

 in the periproct (as for instance by a strong light), it would, 

 according to this rule, tend to move downwards, which is of course 

 impossible ; but as the stimulus never affects all sides quite alike 

 the result is that the Urchin rotates, turning itself ever away 

 from the point of strongest stimulation. In the case of 

 Strongylocentrotus lividus when living on limestone, as on the 

 west coast of Ireland, this results in the animal excavating for 

 itself holes in the rock, where it is safe from the action of the 

 breakers. 2 



But it may be objected that no account is taken in the above 

 description of the action of the " central nervous system," i.e. of 

 the ring and the radial cords, and yet Eomanes found that when 

 they were removed the escape movements could not be carried 

 out. The answer is that the central nervous system is a store- 

 house of tone, not, as in higher animals, a controlling centre for 

 co-ordinating the movements of the spines. When it is removed 

 at first the escape movements can be carried out, but in a day 

 or two all tone in the spine-muscles is lost, and then, since the 

 tone of all is equally low, there is no tendency in those that are 

 stretched to be more responsive than others, and hence the escape 

 movements cannot be carried out. Sea-urchins kept in the tanks 



1 Loc. cit. 2 See note on p. 541. 



