522 ECHINODERMATA ECHINOIDEA chap. 



of an aquarium are apt to lose the tone of their spines owing to 

 the poisoning of the nervous system. 



The central nervous system is, however, the system which 

 controls the movements of the tube -feet. As we have seen, 

 extensions of the radial nerves run to the tip of each podium. 

 Tube-feet are chiefly used in ordinary progression ; when this 

 is quickened the spines come into play exclusively. The 

 extent to which these two organs of locomotion are used 

 varies from genus to genus. Thus Centrostephanus uses its 

 spines a good deal, Echinus and Strongylocentrotus very little. 

 The last-named genus sometimes walks on its tube-feet entirely 

 without touching the ground with its spines. 



The faculty of vision in its simplest form may be defined as 

 sensitiveness to light and shade. Now strong light acts on 

 all Sea-urchins as a general skin irritant. They fly from 

 it towards the darkest corner, and then if it continues the 

 spines rotate. A number of little violet spines on the 

 aboral pole of Centrostephanus longispinosus are especially 

 sensitive to light, and hence are almost constantly in rotation. 

 This is due, according to Uexkiill, 1 to a pigment of a purple 

 colour, which can be extracted by means of alcohol and which 

 is decomposed by light, the products of decomposition being 

 supposed to irritate the* nerves. Centrostephanus when exposed 

 to light becomes darker in colour. This is due to the migra- 

 tion outwards of amoebocytes, which carry a pigment which acts 

 as a screen in order to prevent the valuable visual purple 

 being too rapidly decomposed. Not all Sea-urchins, in fact very 

 few of those living in northern waters, give a reaction to shadow. 

 C. longispinosus is one of the few ; it reacts to a shadow by 

 converging its spines towards it. A much larger number of 

 species inhabiting tropical waters show this reaction. It is 

 entirely stopped if the radial nerve-cords be removed, whereas 

 the reaction to strong light continues. The reaction to shade is 

 strongest after a long previous exposure to light, hence Uexkiill 

 has given the following explanation of it. The continued 

 irritation due to light, having spread to all the spines, eventually 

 reaches the radial cords and is there stored in the bipolar nerve- 

 cells as tone. When the light-stimulus is interrupted some of 



1 "Die Wirkung von Licht und Schatten auf die Seeigel," Zeitschr. fur 

 Biol. xl. 1900, p. 447. 



