. Bulletin - United States National Museum. Science. 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 are the following: Gafillmter multiracbkita (fig. 261, p. 205) rarely has more than 20 arms, yet the brachials are extremely short and discoidal, quite as short as, and freqiiently shorter than, those of the larger C. sen- tosa (fig. 258, p. 205)
. Bulletin - United States National Museum. Science. 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 are the following: Gafillmter multiracbkita (fig. 261, p. 205) rarely has more than 20 arms, yet the brachials are extremely short and discoidal, quite as short as, and freqiiently shorter than, those of the larger C. sen- tosa (fig. 258, p. 205), in which the arms are much more nu- meroiis; but individual variants occur in which the brachials are of the shape normal for 20- armed species. On the other hand, the species of Nemaster (fig. 243, p. 197) and Neocomatella have the brachials rather long for multibrachiate species. In the species of the 10- armed genus Aviphimefra (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 Ileterometra, 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 lIBr series. P3 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 gi-eater and the arm structure approached that seen in
Size: 1250px × 1998px
Photo credit: © Book Worm / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookauthorun, bookcentury1800, bookdecade1870, booksubjectscience
