92 



BULLKTIS" 82, UNITED STATES NATIONAL MUSEUM. 



are exactly like the corresponding arms arising from all the other radials. 

 But in cei'tain of the comasterids tlie arms arising from the one, two, or three 

 posterior radials are modified in various ways and have become curiously special- 

 ized, so that they ai'e no longer strictly comparable to the more primitive arms 

 arising from the one or two anterior radials (fig. 163, p. 86). 



As noticed by P. H. Carpenter these posterior arms (part 1, fig. 45, h, p. 79) 

 differ in numerous particulars from the anterior arms (part 1, fig. 45, a, p. 79). 

 These latter taper very slowly, contain far more numerous brachials, and are much 



longer than the former, 

 while the form of their 

 terminal portions and 

 of the pinnules which 

 they bear is altogether 

 different (figs. 1034. 

 1035. pi. 12). When 

 viewed from the dorsal 

 side the basal portions 

 of the two kinds of arms 

 are precisely similar ; 

 they widen slightly be- 

 tween the first and sec- 

 ond syzygies, remaining 

 uniform until the third 

 syzygy, after which they 

 begin to taper. Up to 

 about the twenty-fifth 

 or thirtieth segment the 

 anterior and the pos- 

 terior arms decrease in 

 width at about the same 

 rate, but from this point 

 onward there is a great 

 difference between them. 

 Fig. 180.— cbnteal pobtion and one post-radial sebibs of specimen of arms borne by the 



Capillaster sentosa FiioM " Albatross " Station 5166. anterior radials taper 



very slowly, the length 

 of their segments increasing considerably while the breadth only diminishes 

 very gradually; at the same time the middle and terminal pinnules in which no 

 gonads are developed become very long and filiform and remain so until the last 

 few brachials, when their length sucfdenly diminishes very considerably. The 

 extreme end of the arm is a growing tip, continually, though very slowly (the rate 

 decreasing with the age of the individual), giving" rise to new brachials, in every 

 way similar to the growing tip of very young arms, or to the growing tip of the 

 arms of Antedon hifda as described by W. B. Carpenter (fig. 1035, pi. 12). 



