192 



BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



most brachials, later on the two proximal brachial pairs, and subsequently on the 

 intermediate brachials; (3) the brachials become much longer; (4) the ends of the 

 brachials become very oblique; (5) the ventral muscle band, a radial continuation 

 of the circumoral muscle band, vanishes and is superseded by a series of paired 

 interbrachial muscles; (6) the articulations between the brachials, up to this time 

 uniform loose sutures, become differentiated into synarthries between the first and 

 second brachials, syzygies between the third and fourth and ninth and tenth 

 brachials, and at regular intervals between the subsequent brachials, and muscular 

 articulations elsewhere; (7) the orals disappear; (8) the radianal disappears; (9) 

 the interradials disappear; (10) the basals withdraw into the interior and become 

 metamorphosed into an internal septum ventral to the chambered organ; (11) the 

 radials become recumbent and form a closely knit radial pentagon to the exclusion 



of all the other calyx plates; (12) 

 the topmost columnal increases 

 enormously in size, spreads itself 

 over the outer surface of the ra- 

 dials and, now become the centro- 

 dorsal, gives rise to numerous 

 cirri; and (13) the larval column 

 is discarded beneath the topmost 

 oolumnal. 



Thus by a sudden and pro- 

 found alteration in the course of 

 development of the arms, pin- 

 nules, calyx, and column com- 

 mencing just as the elements of 

 the second syzygial pair of bra- 

 chials are forming the pentacri- 

 noid of the comatulids changes 

 from a fairly representative mem- 

 ber of the Flexibilia Impinnata into a young comatulid (compare figs. 1217 and 

 1219, pi. 35). 



The oblong proximal brachials, the relatively undeveloped genital pinnules, 

 and the unusually large interval between the first and second brachial syzygies in 

 the arms of the comatulids are therefore all remnants of the ancestral structure 

 as seen in the Flexibilia Impinnata, and the true comatulid brachial structure 

 is not developed until after the second or third brachial syzygy. Since the sec- 

 ondary arms of multibrachiate types are not developed until after the young 

 have become true comatulids, these arms are true comatulid arms throughout, the 

 brachials being all wedge-shaped without special differentiation of the proximal, 

 and the pinnules being all of about the same length and structure and developed 

 in regular sequence. 



The arms of the Atelecrinidce and Pentametrocrinidce. In Atelecrinus (part 

 1, figs. 123, p. 192, and 124, 125, p. 193) and in Atopocrinus (part 1, fig. 227, p. 245) 



232 



FIG. 232. LATERAL VIEW OF SPECIMEN OF STEOTOMETRA 

 HEPBUKXIANA. 



