4 BULLETIN 82, UNITED STATES NATIONAL MUSEUM 



Gislen said that the young of Metacrinus described by himself in 1922 are very 

 instructive on this point. His observations showed distinctly that the formation of 

 IIBr, IIIBr, and IVBr series takes place in the following way: On the simple arms 

 certain pinnules begin to gain strength and to grow. On the sides of the strengthened 

 pinnules new small pinnules arise, and the new arm finally reaches the same length 

 as the main arm. The place for this strengthening of the pinnules is to be found in 

 the region of transition between the large and rudimentary pinnules of the main arm. 

 As that part of the arm provided with the latter is shorter the younger the individual 

 is, it is clear that the arm ramification for the formation of IIBr axillaries will occur 

 fairly near the tip of the arm. Therefore the main arm and the new arm are not very 

 unequal in length there, whereas the new arms on the IIIBr axillaries, and to a still 

 greater extent on the IVBr axillaries, are very different in length at earlier stages. 

 Thus the lengths of a main and a side arm on a IIBr axillary (Metacrinus interruptus, 

 specimen 16) are 2 and 3 mm. respectively; of two young arms from a IIIBr axillary 

 (M. interruptus, specimen 17) 5 and 1 mm.; of arms from a IVBr axillary (M. inter- 

 ruptus, specimen 13) 12 and 2 mm., and (M. nobilis tenuis, specimen 9) 5 and 1.2 

 mm., or 3 and 0.8 mm. 



Here, said Gisle"n, the factor of obstruction only succeeds in acting temporarily 

 on the pinnule that is destined to become an arm. Thus arm ramification in Meta- 

 crinus is more direct and primary than in the comatulids a pinnule is strengthened, 

 it ramifies, and then it grows until it is equivalent to the main arm. 



In the pentacrinites there is regeneration of broken arms, just as in the coma- 

 tulids, and as in the latter the fracture is most often at the syzygies. When a post- 

 radial series has been broken before a last axillary, regenerates appear that are like 

 the augmentative regenerates of the comatulids. It is only by following series in 

 different stages of development that the difference in principle between comatulids 

 and Metacrinus in the way their arms increase in number can be ascertained. Thus 

 the pseudo-augmentative arm regeneration of which examples have been given is only 

 of a reproductive nature. 



In this connection Gisle"n pointed out with regard to the comatulids that repro- 

 ductive arm regeneration appears in them also. It is necessary to avoid interpreting 

 an axillary regenerate as evidence of augmentative regeneration, but the comatulids 

 undoubtedly have augmentative arm regeneration as a means of increasing the num- 

 ber of their arms, whereas in Metacrinus the method is more direct and primitive. 

 A forerunner of augmentative arm regeneration is reproductive arm regeneration, 

 which occurs in Metacrinus together with the primitive method of augmentation. 



In the more specialized comatulids the strengthening of a pinnule into an equiva- 

 lent of the main arm never occurs normally, except possibly in Comatula etkeridgei 

 (the young of C. rotalaria). In these arm regeneration becomes augmentative as the 

 surfaces of syzygies in the proximal parts of the arms acquire the power of forming 

 axillaries with a greater number of arm branches than the lost portion of the post- 

 radial series had. The apparently simpler postradial series of the comatulids must be 

 considered as potentially forked, but the ramification does not materialize because 

 the factor of obstruction permanently restrains the efforts of the pinnules to develop 

 directly into arm branches. This ramification does not appear until after the break- 



