78 STUDIES IN THE DEVELOPMENT OF CRINOIDS. 



columnars, as is knowTi 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 api^carance of the 

 oral pinnules observed in Com'psovietra (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. 



Q 



Fig. 9. — Four Buccessive stages in the development of the cirrus joints of ArUcdon bifida. X 180. 



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 of the Crinoid is really a sympodium 

 formed of a succession of the stronger members of successive dichotomies." 



There is, however, one serious objection to this representation of the 

 pinnule and arm formation: that it has no foundation in reality. Bcj^ond 

 the axillary there is (in 10-armed Comatulids) no real bifurcation of the arms; 

 the pinnules appear from the beginning as side branches of the arm, not as 

 equivalents of the end of the arm. On a closer examination of the growing 

 arm-point it is easily seen that the young pinnule-joints are much smaller 

 than the arm-joints, and they are from their first appearance lying at an 

 angle to the growing arm-point, which latter continues to grow in a straight 

 line, new joints constantly forming at its tip. (Text-figure 10.) It is true, 



