190 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



In the 10-armed comatulids there is a very appreciable difference between the 

 arms as far as the second or third syzygial pair (brachials 9+10 or 14+15) and 

 beyond that point (see figs. 220, p. 168; 221, p. 171; and 222, p. 173). The char- 

 acteristic features of the first pair of brachials have already been discussed. Be- 

 tween the first and second syzj'gial pairs the brachials are short, usually about 

 twice as broad as long and more or less oblong or wedge-shaped : beyond the second 

 syzygial pair they become triangular, about as long as broad, distally elongate 

 wedge-shaped, and terminally much elongated with their ends more or less at 

 right angles with the longitudinal axis of the arm. 



There is a corresponding difference in the pinnules. The two lowest pinnules — 

 the first (P,) on the outer side of the distal element of the first brachial pair, 

 the second (Pa) on the inner side of the distal element of the second brachial (first 

 syzygial) pair — are typically much longer, stouter, and better developed than 

 those succeeding, and the articulations between their segments, especially between 

 the first and second, approach more or less closely the muscular articulations be- 

 tween the brachials. Following these to the second or third syzygial pair there is 

 a series of pinnules which are short and more or less rudimentary, after which 

 the pinnules become greatly elongated and extremely slender. 



All the evidence seems to indicate that in the comatulid arm there is a marked 

 increase in the rate of growth after the second syzygy, which reaches its full effect 

 at or shortly after the third syzygial pair. 



The following facts appear to show that this is the case : The brachials become 

 much longer. The pinnules become much longer, with greatly elongated segments 

 On the brachials before the second syzygial pair the produced and spiny distal 

 edges, when developed, stand up at right angles to the longitudinal axis of the 

 arm, and are duplicated on the proximal edges; but beyond the second syzygial 

 pair the production of the proximal edges of the brachials entirely disappears, and 

 that of the distal edges becomes accentuated and leans gradually distally, over- 

 lapping the base of the succeeding brachials. In species in which all the brachials 

 are strongly carinate the keels on the earlier wedge-shaped brachials are of uni- 

 form height from end to end ; as the brachials become elongated the keels transform, 

 by the sinking of the proximal and the rising of the distal portion, into a triangular 

 overlapping spine, of which the base coincides with the mid-dorsal line of the 

 brachial. In species in which the dorsal surface of the brachial bears nmnerous 

 very fine spines those on the proximal wedge-shaped brachials are without definite 

 arrangement and occupy a broad median band with parallel sides ; on the elongate 

 brachials following they occupy a triangle, of which the base is the distal dorsal 

 edge and the apex lies at the middle of the proximal edge, and are arranged in 

 longitudinal rows. Beyond the second the two elements of the syzygial pairs 

 become united at an earlier stage, so that the hypozygal loses its individuality and 

 the pair as a whole becomes similar to a normal brachial. 



In the secondary arms of the multibrachiate species, which replace the 10 

 primary arms, the structure is much more uniform than in the arms of the 10-armed 

 species, or in the arms of their own 10-armed j'oung. There is a greater uniformity 

 in the shape of the brachials, all of which are typically wedge-shaped, and the 



