458 Charles R. Stockard 



appendages regenerate more rapidly when three or four are re- 

 moved than when one or two are cut away. Zeleny's final meas- 

 urements were made on only 36 animals and these had regenerated 

 very short arm buds. After 46 days only a few of his individuals 

 had new arms even 5 mm. in length. These facts tend of course 

 to increase the probability of error. 



The experiments now to be recorded continued for 49 days at 

 the end of which time 135 specimens were measured. The aver- 

 age length of the new arms ranged from 29 to 46 mm. The error 

 in measurements is greatly decreased in arms of such length. 

 (An error of I mm. here would equal an error of only o.i mm. in 

 Zeleny's measurements. A i mm. error is unlikely to occur but 

 errors of o.l mm. in measuring brittle-star arms are difficult to 

 avoid.) 



Fortunately two species of ophiurans were employed in the 

 experiments since it happens that they differ slightly in their 

 responses to different degrees of injury. The case of Ophiocoma 

 riisei, a large black spiny form with reddish tube feet, may be con- 

 sidered first. 



Perfect animals were selected and grouped into five lots the 

 individuals of the lots being of the same average size. All arms 

 were amputated i cm. out from the disk with sharp scissors. The 

 first lot had one arm cut from each brittle-star, in the second two 

 arms were cut off, the third lost three arms, the fourth four and 

 all five of the arms were cut from each individual in the fifth lot. 

 After 49 days the specimens were expanded and killed in fresh 

 water and chloretone and preserved in alcohol. The new arms 

 plus the old I cm. stump were not so long as the original arms nor 

 were they equal to the old ones in thickness. 



A correct comparison of all the groups is made by considering 

 the averages of actual new arm lengths shown in Table IX. Origi- 

 nally the average diameters of all the lots were practically the same, 

 therefore, the arm lengths are to one another as the quotients 

 (specific amount of regeneration) would be if they were divided 

 by the originally equal diameters. I have calculated the specific 

 amounts on the basis of final diameters merely to show the erro- 

 neous impression obtained by using such a method. 



