GENERAL PART. 



79 



as Carpenter says, that one of the rami grows faster than the other, but it 

 is the pinnule which grows faster than the arm-joint and soon reaches 

 beyond it, but it never "takes a Hne continuous with that of the axis of the 

 arm," as Carpenter thought it did. The Crinoid arm is thus no sympodium, 

 but in reality what it has the appearance of being — a single arm with small 

 alternating side-branches, the pinnules. 



The place of formation of new joints in the growing pinnules was not 

 made out with certainty by Carpenter; he came to the conclusion that "it 

 may be safely assumed that they are not developed at the terminations of the 

 pinnules, since their pecuHar terminal 

 hook is formed when as yet the segments 

 are few in number." The new joints must 

 then be intercalated "either between the 

 basal and the second segment, or between 

 the penultimate segment and the terminal 

 claw-bearing segment. Since no such 

 traces of incompleteness present them- 

 selves in the segments which follow the 

 basal as would justify the former supposi- 

 tion, we seem compelled to adopt the 

 latter ; and it is not a little curious that the 

 increase in the number of segments in the 

 Stem, the Dorsal Cirrhi, the Arms, and 

 the Pinnules should thus take place in 

 different modes — the new segments mak- 

 ing their appearance in the Stem immedi- 

 ately beneath its highest segment, in each 

 Dorsal Cirrhus at its base, in each Arm 

 at its termination, and in each Pinnule at 

 the base of its terminal segment." (W. B. 

 Carpenter, op. cit., pages 747-748.) 



That Carpenter is mistaken in this point is easily ascertained by a closer 

 examination of the newly formed pinnules at the point of the arm. The new 

 joints are formed at the tip of the pinnule, and the terminal hook is the last 

 to form. All the joints are formed in the course of a very short time, while 

 the pinnule is still very short; already in the third pinnule from the point 

 they are all formed, further growth of the pinnule depending on the lengthen- 

 ing of the joints. (Te.xt-figure 10.) 



Thus of the statements quoted above from Carpenter regarding the mode 

 of formation of new joints in the arms, the pinnules, the stem, and the cirri, 

 only the first is correct; in the arms the new joints appear at the termination, 

 as he says; but in the pinnules they do not develop between the two last 

 segments, but at the termination ; in the cirri, not at the base, but likewise 



Fici. 10. — Point of an arm of .'1 nte^hn bijihi, 

 showing the first rudiment of the outer pinnule 

 and second pinnule with 9 joints, the lower- 

 most of which is almost fully formed. X 180. 



