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VOL. XXXV. 



radial) joint, and Z 2 the next following; succeeding Z„ are two joints 

 united by syzygy ; now Z 2 always has distally an oblique muscular 

 articulation; but a 4 (3+4) or 4 (1+2; 3+4) second post-radial 

 division series always has the two component parts separated by a 

 straight muscular articulation; the explanation appears to be that 

 when an oblique muscular articulation on the distal face of Z 2 , 

 through autotomy taking place beyond it, comes to occupy the posi- 

 tion of a straight muscular articulation, the dominance of the latter 

 asserts itself, and the oblique muscular articulation of the young 

 gradually transforms into the straight muscular articulation of the 

 adult. 



Now, since the second (and following) post-radial division series 

 of the comatulid arm are frequently doubled, appearing as 4 (3+4) 



Fig. 24. — Comastkrid.e ; Com aster. 



(figs. 15, 16, 19, and 20) or 4 (1+2; 3+4) (fig. 23) instead of 2 

 (fig. 22), we should expect that the first post-radial division series 

 would occasionally be doubled, since it is morphologically comparable 

 to the more distal division series, and we find that such, though 

 rarely, is the case; for Carpenter records that in one "Antedon" 

 that passed through his hands " one of the rays consists of five joints, 

 the axillary being a syzygy." 



° Challenger Reports, XI, Zoology, p. 51. 



