102 Papers from the Marine Biological Laboratory at Tortugas. 



is most striking. No doubt the existence of localized stages in the series of 

 vertebra of the ophiuran arm will be easily demonstrated when the details 

 of arm-structure are worked out for a large number of species and genera. 



The development of the young vertebra into the adult deserves a further 

 word. It has already been pointed out that during this development growth 

 longitudinally is very slow as compared with the lateral and vertical growth. 

 Attention may now be given to the important question as to what processes 

 of the adult vertebra are the first to develop and which are the last. In a 

 very young vertebra the alae are the only evident projections, but in one a 

 little older (pi. i, fig. 8), the aboral hypapophyses are well marked, the 

 zygosphene is defined, and the aboral end of the protapophysis can be 

 distinguished; there is also an indication of the epanapophysis. In the 

 vertebra of the segment, 5 joints nearer the disk (1. e., the fiftieth), the upper 

 alar ridge is distinct and the protapophysis is fully defined. In the vertebra 

 of the fortieth segment the parapophyses show and the aboral hypapophyses 

 are very prominent. In the vertebra of the thirtieth segment the first 

 indications of the epapophyses are seen in the line of distinction between the 

 protapophysis and the alar ridges. In the vertebra of the twentieth seg- 

 ment this distinction is so far completed that the aboral epapophyses and 

 the zygantrum are well defined. 



Before leaving the subject of the vertebrae, a word may be said as to 

 their articulation with each other. The ala of course furnish the chief 

 surface for muscular attachment, but the parapophyses and hypapophyses 

 are important points of attachment. The zygosphene fits into the zygo- 

 treme of the next distal vertebra and is supported on each side by a zyga- 

 pophysis. The aboral margin of the protapophysis fits into the space below 

 the epanapophysis and above the zygapophyses, and the aboral hypa- 

 pophyses fit into the space below and outside the adoral pair. So while the 

 zygosphene and zygotreme furnish what might be called the axial joint, 

 which is thus of the ball-and-socket type, too great vertical freedom is pre- 

 vented by the protapophysis and epanapophysis, while excessive lateral 

 movement is impeded by the aboral and adoral hypapophyses. 



ARM-PLATES. 



The sequence of formation of the arm-plates has been correctly stated 

 by Ludwig and by Fewkes, their observations on Amphipholis being fully 

 confirmed by mine on that genus and on Ophiactis. The 2 side arm-plates 

 appear simultaneously, one on each side, directly back of the terminal 

 plate, and are the first indication of the formation of an arm-segment. 

 They very quickly meet in the mid-ventral line and a little later in the mid- 

 dorsal line. They are followed at once by the rudiment of the under 

 arm-plate, which arises directly in front of, or distal to, their ventral line 

 of division. The under arm-plate is soon followed by the upper one, which 

 is formed in a corresponding position on the dorsal side of the arm. In a 

 normally developing young arm, the fourth segment is generally the first 



