130 



BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



shaped than 10-armed forms, the shape of the brachials is not always strictly pro- 

 portionate to the number of the arms. 



The chief exceptions to the broad generalization that the larger the number 

 of arms the shorter and more wedge-shaped the brachials arc the following: 



CapUlaster multi I'ad-'uita (fig. 261, p. 205) rarely has more than 20 arms, yet the 

 brachials are extremely short and discoidal, quite as short as, and frequently shorter 



than, those of the larger ('. sen- 

 tosa (fig. 258, p. 205), in which 

 tlic 1 arms are much more nu- 

 merous: but individual variant- 

 occur in which the brachials are 

 of the shape normal for 20- 

 armed species. 



On the other hand, the 

 species of .\<'tnst,'r (fig. 243, 

 p. 197) and Neocomotrtla have 

 the brachials rather long for 

 multibrachiate species. . 



In the species of the 10- 

 armed genus Amphimstra (fig. 

 259, p. 205) the brachials are ex- 

 tremely short and narrowly ob- 

 long in dorsal view, in this 

 agreeing with the brachials of 

 several species of the closely 

 allied genus Flettrometra, in 

 which there are fewer than 20 

 arms. 



A striking characteristic of 

 many of the species of Hetero- 

 metra is the irregularity of the 

 number of arms on the differ- 

 ent rays, indicating a marked 

 instability in the arm division. 

 Apparently in correlation with 

 this, individuals are frequently 

 met with in which the forma- 

 tion of additional arms is tak- 

 ing place after the full size has been reached, indicating an exceptionally strong 

 latent tendency toward increase in the number of arms. As has been mentioned, 

 there is commonly an additional axillary on the inner branches from each II Br 

 series. P 3 is the longest, and is usually best developed on the inner arms. All of 

 these features, as well as the very short brachials, appear to suggest that these 

 species are descended from others in which the arm number was much greater and 

 the arm structure approached that seen in the genus Himernmctra (fig. 260. p. 2051. 



PIG. 200. LATERAL VIEW OF SPECIMEN OF NEOMETRA DIANA. 



