128 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



In a number of species, especially in the Ptilometrinse, the brachials carry 

 in the mid-dorsal line a prominent raised keel. On the proximal wedge-shaped 

 brachials this keel is of uniform height, but as the brachials increase in length 

 the distal outer portion becomes higher and higher and the proximal portion lower 

 and lower, so that a high triangular overlapping spine results, the base of which 

 coincides with the mid-dorsal line of the brachial which bears it, while the apex 

 lies more or less beyond the level of the proximal end of the succeeding brachial. 



The differences between the elements of the first brachial pair and the 

 brachials following the first syzygial pair have already been considered. The 

 wedge-shaped proximal brachials commonly are more or less intermediate in 

 their characters between the first brachial pair and the first syzygial pair and 

 the brachials beyond the latter, though always much more like the distal brachials. 



Distributed throughout the arm among the brachials of the usual type are 

 pairs of brachials united by syzygy. In the proximal portion of the arm these 

 syzygial pairs are always somewhat higher than single brachials, and in some 

 cases are twice as high, so that both the hypozygals and the epizygals are indi- 

 vidually approximately the equals in size of the ordinary brachials: but distally 

 both the hypozygals and the epizygals gradually decrease in relative height, so that 

 finally in the outer part of the arm the syzygial pairs come to have quite the 

 appearance and size of ordinary brachials. 



In species in which the distal edge of the brachials is strongly produced into 

 an overlapping and spinous distal edge or other form of eversion, or in which the 

 brachials bear dorsally a prominent spine, the hypozygals of the syzygial pairs 

 show a similar though less prominent modification at the line of syzygial union : 

 but distally this modification of the distal edges of the hypozygal gradually de- 

 creases, so that in the outer part of the arm the distal edge of the hypozygal is 

 quite unmodified. 



In the very young, as in regenerating arms or at the growing arm tips, the 

 elements of the pairs which are later to become united by syzygy are in no way, 

 either through shape, size, or method of articulation, to be differentiated from the 

 other brachials. This differentiation is effected at a comparatively late stage of 

 brachial development. 



In all of the more primitive comatulid types, in nearly all of the 10-armed 

 species, and in the 10-armed young of the multibrachiate forms, the syzygial pairs 

 are regularly spaced, alternating with single brachials or, usually, separated by 

 from two to four of them, the number being definite for each species. In the 

 proximal portion of the arm the first syzygy is between the third and fourth 

 brachials, the second between the ninth and tenth, and the third between the four- 

 teenth and fifteenth, or, much less commonly, between the fifteenth and sixteenth 

 (fig. 1018, pi. 7). In the outer part of the arm the number of brachials of the 

 ordinary type between successive syzygial pairs is more or less correlated with the 

 size of the species, very small species commonly having the syzygial pairs sep- 

 arated by single brachials, larger species having them separated by two or three 

 brachials, and the largest species having three or four brachials between the 

 syzygial pairs. 



