^6 THE AMERICAN ARBACIA 



of Reaumur, who noticed that they used their spines as feet and mar- 

 velled at the number of muscles it must take to move 2,100 spines. 



Arbacia punctulata can "right" themselves, i.e., turn over to the ven- 

 tral side when placed on the dorsal side, easily and quickly, both large 

 and small ones. In a flat-bottom glass dish, they raise themselves to 

 their side sometimes within two minutes and then fall to the ventral 

 side quickly by gravity (E. B. H.). Von Uexkiill (1909, p. 106) says 

 A, lixula cannot turn over on a flat surface, only on a slanting surface, 

 but A. punctulata seems to have no difficulty, if in water. It cannot turn 

 over when out of water. 



The righting movements for Echinus esculentus and P. lividus have 

 been described in detail with illustrations by Romanes and Ewart 

 (1881) and Romanes (1885), and for Arbacia lixula by von Uexkiill 

 (1909, p. 106). Parker (1927) and Parker and Van Alstyne (1932) 

 have described locomotion and righting movements in Echinarachnius. 



Though locomotion is usually accomplished by the tube feet and 

 spines, it can also be effected by the lantern as described by Romanes 

 and Ewart (1881) and by Gemmill (191 2) in Echinus. The lantern may 

 be used both for progression and rotation when the animal is out of 

 water. 



