DIRECTION OF LOCOMOTION OF THE STARFISH 



13 



expermients were not accurately measured nor preserved, but 

 in order to ascertain whether perhaps this condition of shorter 

 'posterior' arms held generally, though in a less striking degree 

 (since otherwise it would have been noted) careful measurements 

 of the length of arm have been made on a series of 116 specimens 

 of Asterias forbesi from the Woods Hole region, and of about 

 the size of those used in the experiments. A brief statement 

 of the results of these measurements will suffice for the present 

 discussion. The specimens measured were a selected sample 

 only in that individuals about the size of those used in the exper- 

 iments were included and that those having arms of obviously 

 disproportionate lengths (probably regenerating arms) were ex- 

 cluded. This was done also in selecting individuals for the 

 experiments. Nevertheless, in measuring this somewhat re- 

 stricted lot, one was most impressed by the considerable varia- 

 tion in arm length and its apparent irregularity as to position. 

 It was quickly discernible that no one arm, nor pair of arms, 

 was regularly the longest or the shortest. The number of times 

 each arm occurred as the longest or as one of the two longest 

 (in cases where the longest two measured the same) and as the 

 shortest or one of the two shortest, was as follows: 



These figures correspond in a general way to the results on 

 locomotion, and in so far would appear to show a correlation 

 between length of arm and the frequency with which it moves 

 in advance. Thus it will be observed that arms c and d are 

 longest the least number of times, while arm a is most often 

 longest (or one of the two longest). Conversely arms c and d 

 are the shortest more often than the others, and arms e and a 

 are shortest the least often. This may be seen most readily 

 where the figures are plotted as in figure 6, which should be 

 compared with figure 4. It will be noted that except for the 



