. Carnegie Institution of Washington publication. . 78 STUDIES IN THE DEVELOPMENT OF CRINOIDS. columnars, as is known from Bury's researches, at first have the shape of a half-moon and gradually assume the shape of a ring; the brachials and also the pinnule joints originate as a simple transverse rod, which never assumes the shape of a ring (see page 29). The order of appearance of the pinnules is this: the first to form is that on about the twelfth arm-joint; then follow some more pinnules along the growing-point of the arm, and it is only after some 5 or 6 arm-pinnules have been formed that
. Carnegie Institution of Washington publication. . 78 STUDIES IN THE DEVELOPMENT OF CRINOIDS. columnars, as is known from Bury's researches, at first have the shape of a half-moon and gradually assume the shape of a ring; the brachials and also the pinnule joints originate as a simple transverse rod, which never assumes the shape of a ring (see page 29). The order of appearance of the pinnules is this: the first to form is that on about the twelfth arm-joint; then follow some more pinnules along the growing-point of the arm, and it is only after some 5 or 6 arm-pinnules have been formed that the oral pinnules begin to appear, that on the second arm-joint being the first of them. Whether the order of appearance of the oral pinnules observed in Compsometra (see page 28) has a more general application remains uncertain, but it is evidently a general rule among Comatulids that the oral pinnules do not appear till after some of the arm- pinnules have been formed. This was previously observed by both W. B. Carpenter and Sars. O FIG. 9.—Four successive stages in the development of the cirrus joints of Antcdon bifida. X180. Regarding the development of the pinnules, W. B. Carpenter states (op. cit., page 734) that they appear at the growing extremity of the arm "which now presents a bifurcation, the two rami being in the first instance almost equal .... One of these rami, however, grows faster than the other, and soon takes a line continuous with that of the axis of the arm, from which the other diverges at an acute angle; so that the former comes to be the proper extension of the arm, whilst the latter soon takes on the characters of a pinnule"—and so on, the point of the arm constantly bifur- cating, the branch to the right and left alternately developing into a pinnule, the other branch taking on the character of the arm. This is very clearly expressed thus by MacBride, in his "Text-book of embryology," (page 557): "The apparently single arm
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