MONOGRAPH OF THE EXISTING CBINOIDS. 201 



These arms are similarly uniserial but are developed upon axillaries, and there- 

 fore always paired, lying on one side or other of the median line, never in the 

 middle. In the same way at the stage when the arms of the pentacrinoid are 

 equivalent to the arms of the Flexibilia they are always two in number and lie 

 one on either side of the mid-ambulacral line. In the posterior arms of certain 

 of the Comasteridse the terminal brachial is an axillary bearing two similar 

 pinnules side by side (fig. 1034, pi. 12, and part 1, fig. 47, p. 81). 



Viewed in the light of the conditions found in the echinoids and starfishes 

 this doubling of the elements in the ambulacral series in the crinoids is easily 

 explained as the retention of the fundamentally biserial character of the echino- 

 dermal ambulacral plates. 



The comatulid arms are elongate structures developed in the midradial line 

 from two columns of plates, one situated on either side of it, which have combined 

 themselves in such a way as to form a secondarily uniserial branched appendage. 



Nonmuscular articulations. The nonmuscular articulations are strictly com- 

 parable with the sutures between the calyx plates and between the radials, and occur 

 only between elements which originate as longitudinal pairs, as in the arm and 

 column, or as longitudinal series of exactly similar ossicles, as in the pinnules and 

 cirri. Ossicles originating as transverse pairs are always united to the preceding 

 ossicle by muscular articulation. But while the elements of transverse pairs are 

 united to the preceding ossicle by muscular articulations, the elements of each pair 

 immediately following an axillary are commonly more or less bound together by 

 suture. In each species all of the transverse pairs of ossicles immediately following 

 the outer axillaries are united to approximately the same extent as those immedi- 

 ately following the IBr axillary, and in different specimens of the same species, 

 where there is considerable variation in the latter, there is a similar correlated varia- 

 tion in the former. Increase in size or in the number of the arms induces a broaden- 

 ing of the IBr, and a closer lateral union, and the same feature is projected upon 

 all the transverse pairs immediately following axillaries farther out in the post- 

 radial series. As a general rule the IBr, are united for a slightly greater propor- 

 tion of their interior length than the IIBr,, and the IIBr, than the IIIEr,, and so 

 on, so that there is a very gradual lessening of the completeness of this internal 

 union distally. The graduation is commonly so slight as to be barely appreciable, 

 except in those species with a very large number of arms. 



In the pentacrinoid larvae of the comatulids before the appearance of the first 

 pinnules all the articulations, both of the column and of the arms, are nonmuscular. 

 Specialization in the comatulid arm is in the direction of the gradual elimination of 

 the nonmuscular articulations, which are replaced by unions of the muscular type. 



Up to and including the first syzygial pair of the free undivided arms all the 

 ossicles beyond the radials are united in pairs by nonmuscular articulations, 

 synarthries between the two ossicles immediately following each axillary, and 

 syzygies between the outer pair in the division series in which two pairs are 

 developed. 



