THE GEOTROPISM OF THE SEA-URCHIN 

 CENTRECHINUS. 



G. H. PARKER, 

 ZOOLOGICAL LABORATORY, HARVARD UNIVERSITY. 



The long-spined sea-urchin, Ccntrcchinus antillarnm (Phil.), is 

 abundant on the flats near the Miami Aquarium in Biscayne Bay, 

 Florida. It is a common object of interest in the aquarium tanks, 

 where it is found climbing up the stone-covered walls or perched 

 on the top of the submerged rock-work. When transferred to a 

 large glass jar filled with quiet or running sea water, it immediately 

 starts to climb up the sides of the vessel, and each time it is 

 loosened and dropped to the bottom it renews this activity. This 

 response occurs in the dark as well as in the light, and in vessels 

 the water of which is cut off from the air by a glass plate as well 

 as in those whose contents are exposed to the air. In other words, 

 Centrechinus climbs upward, not because of light or of access to 

 oxygen, but in response to gravity. It is a strongly geonegative 

 animal. 



Centrechinus belongs to the group of regular sea-urchins, but, 

 like most representatives of this group, its radial symmetry is 

 disturbed by the single madreporite which occupies the aboral end 

 of one of its five interradial zones. Thus a structural axis of 

 orientation is established. As Centrechinus is remarkably active 

 in its geotropic responses, it is a favorable form with which to test 

 orientation in locomotion. Does it move as bilateral animals al- 

 most always do, with one end of a definite axis constantly for- 

 ward, as, for instance, the one marked by the madreporite, or may 

 any of its numerous axes serve as a line of progression? This 

 general question has already excited the attention of a number of 

 students of the echinoderms. 



Jennings (1907, p. 155) states that the California starfish, 

 Asterias fon-cri, "may move with any one of its rays in the lead, 

 or with any interradius in advance, or indeed in any intermediate 

 direction, so that its possibilities as to variations of direction of 



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