I in I. l.l. I l\ 82, UNITED STATES NATIONAL MUSEUM 



Gislen said tlint tlic young of Metacriwua described by himself in L922 are very 

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



II Br. HIBr. and [VBr series takes place in the following way: On the simple arms 

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

 pinnules aew 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 thai pari of the arm proi ided with the latter is shorter the younger the individual 

 is. it is dear 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 



1 1 ji 1 in length there, whereas the new arms on the III Br axillaries. and to a still 

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

 Thus the lengths of a main and a side arm on a 11 Br axillary {Metocrinus interruptus, 

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

 (M. interruptus, specimen L7) 5 and 1 mm.; of arms from a IVBr axillary (M. inter- 

 ruptus, specimen 13) 12 and 2 mm., and (.U. nobili* tauils, specimen it) .5 and 1.2 

 mm., or 3 and 0.8 mm. 



Here, said Gislen, 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 Metocrinus in the way their arms increase in number can be ascertained. Tims 

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

 of a reproductive nature 



In this connection Gislen 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 M<!'icrinus the method is more direct and primitive. 

 A forerunner of augmentative arm regeneration is reproductive arm regeneration, 

 which occurs in Metocrmus 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 Cimiattila etheridgei 

 (the young of C. rotatoria). 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 Beries had. The apparently simpler postradial series of the comatulids must be 

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

 the factor <d nhsi ruction permanently restrains the efforts of the pinnules to develop 

 directly into ami branches. This ramification does not appear Until after the break- 



