100 Papers from the Marine Biological Laboratory at Tortugas. 



fig. 4), but on a growing jaw the youngest tooth, the lowest, is distinctly the 

 smallest. 



ARM-BONES OR VERTEBRA. 



Although Sirproth (1876) has figured the vertebrae of Ophiactis virens, 

 it has seemed desirable to give a series of illustrations of the vertebrae in 

 savignyi, not merely because they are obviously different from those of 

 virens, as shown by Simroth, but because the series in each arm, taken as a 

 whole, reveals stages in development which are of particular interest. As 

 the origin and growth of the vertebrae are in all essentials exactly as Ludwig 

 and Fewkes have described in Amphipholis, there is no occasion to go into 

 great detail here. The growing tip of the radial water-tube is protected by 

 a cylindrical plate or, better, a calcareous cylinder, back (proximal) of which 

 arise all the calcareous structures of the arm. The first plates to appear 

 are the side arm-plates, but no sooner are they well under way than the 

 rudiment of the under arm-plate appears followed by the pair of calcareous 

 rods which are to form the vertebra, lying side by side, above (dorsal) the 

 water-tube. They develop very rapidly and soon the proximal ends are in 

 contact with each other (pi. i , fig. 6). New joints are arising distal to them, 

 and by the time half a dozen joints have appeared the two halves of the 

 vertebra are in very close contact; when 10 joints have formed it will be 

 found that the first-formed vertebra is now a single, well-formed, bilateral 

 structure, which can hardly be separated into its component halves. At 

 this time the vertebra is much longer than wide, a little wider than high, 

 and narrower distally than proximally. It soon begins to broaden at the 

 distal end, a sort of knob or projection forming on each side (pi. i, fig. 8). 



Further development is chiefly along three lines: rapidly increased pro- 

 portional breadth, greatly increased height, and growth of spurs, knobs, 

 ridges, and hollows. The increased breadth is easily seen by comparing 

 the vertebra of the thirtieth segment of an adult arm (pi. i, fig. 11) with that 

 of the fifty-fifth (pi. i, fig. 8) ; while the length has only increased 25 per cent, 

 the breadth has increased 180 per cent; or compare a fully developed 

 vertebra, as in the tenth segment (pi. i, fig. 14), with the young one of the 

 fifty-fifth segment, and it will be seen that while it is only twice as long, it 

 is more than six times as wide. The increased height is easily shown in the 

 same way; thus although in segment 10 the vertebra is about 8 per cent 

 higher than long (pi, i, fig. 18), in segment 55 it is more than three times 

 as long as high. The development of spurs, knobs, ridges, and hollows is 

 equally remarkable and is easily understood by an examination of the 

 figures given. Unfortunately no detailed description of an ophiuran verte- 

 bra, with technical names for the different parts, has ever been published, 

 Ludwig, Lyman, and others making use of descriptive phrases for the 

 various parts. Since the structure of the vertebrae will undoubtedly play 

 a more important part in the classification of ophiurans in the future than 

 it has in the past, it is desirable to have a uniform nomenclature for the 

 various parts. 



