98 



Papers from the Marine Biological Laboratory at Tortugas. 



m 



DOES ONE RAY HAVE GREATER FUNCTIONAL VALUE IN 

 LOCOMOTION THAN ANOTHER? 



The experiments of Preyer (1886-1887), in which he tested several 

 species of starfish to see if one ray had greater functional value than 

 another in locomotion, seem to answer the question in the negative except 

 in the case of a specimen of Astropecten pentacanihMS , which showed a 

 decided tendency to use ray 4 {d of Jennings's paper and my own) as 



director. In general, however, 

 Preyer concluded that one ray 

 has no greater functional value 

 in locomotion than another 



(fig. I). 



While Jennings (1907) in 

 his work on the starfish has 

 not taken up this particular 

 problem, he has made a rather 

 extensive study in order to 

 determine if Aster i as forreri de 

 Loriol tends to right itself on 

 any special pair of rays and he 

 concludes that the rays lying 

 close to the madreporic plate 

 are more often used than any 

 others. 



Bohn (1908) finds that 

 specimens of Aster i as rubens 

 show "une sorte de preference 

 pour certains bras," especially 

 when the individuals are of a large size. The rays j (c) and 5 (e) of one 

 individual were used most frequently as directors in locomotion. Bohn, 

 however, does not draw the conclusion that A. rubens shows any very 

 definite habit in this respect. 



Recently, in a preliminary report (1909), I stated that Echinaster 

 crassispina does not show any tendency to use a special ray or pair of 

 rays as a director. This conclusion was based on a considerable number 

 of observations with directive light excluded, but the work was not a 

 careful statistical study such as that undertaken by Cole (19 10). The 

 latter finds that Asterias forbesii, in the absence of directive stimuli, 

 moves most often with the ray (e) in advance and it is possible that if 

 Echinaster crassispina is subjected to similar tests a tendency to move 

 with a certain ray in the lead may be shown. 



Fig. I. — Diagram of a starfish in the normal position, i.e., 

 with aboral surface uppermost. The labeling of the 

 rays by numerals is that used by Preyer and Bohn; 

 the labeling of the rays by letters is that used by 

 Jennings, Cole, and the writer. The madreporic plate 

 is indicated by the black spot and is labeled m. 



