98 Papers from the Marine Biological Laboratory at Tortugas. 



ment of 3 new radial canals. As no 4-armed or 7-armed specimens were 

 seen (excepting of course the 4-armed complements of 2-armed individuals), 

 it would seem to be true that the presence of 4 arms on a regenerating half 

 always prevents the growth of the third new radial canal, while the presence 

 of only 2 arms always prevents the suppression of the third new arm. 

 In very young regenerating individuals, the difference in size between the 

 third new arm and its two fellows is usually quite marked (pi. i, fig. l). 



As Simroth, Fewkes, and Ludwig have all pointed out, the development 

 of the skeletal pieces of the new arms takes place rapidly at the tip of the 

 growing radial canal, forming a covering about it and, as it gives rise to 

 pairs of podia, forming the vertebrae and arm-plates of successive joints. 

 In Ophiactis the podia are not peculiar in any way, but are simple, slightly 

 tapering, blunt tubes with thin, smooth walls. We may therefore turn 

 our attention to the skeletal plates. 



DISK-COVERING. 



In the youngest specimen found the disk measured a millimeter across, 

 but it was only semi-circular, since division had already occurred at least 

 once (pi. I, fig. 2). That not more than one division had occurred is indi- 

 cated by the arrangement of the plates on the disk. The older half of the 

 animal is easily distinguished from the new one developing, the size of the 

 plates in the two areas contrasting sharply. The older half is covered by 

 3 pairs of radial shields, a central plate, 3 radial plates, and 6 interradials. 

 On the developing half-disk it is less easy to identify the plates, but there 

 seem to be 3 pairs of radial shields, separated from each other by interradial 

 plates. The arrangement of the plates on the older half corresponds well 

 with that which Ludwig (1899) figures for specimens of Ophiactis asperuJa 

 with the disk only 0.68 to 0.87 mm. across. The only difference is in the 

 relative sizes of the different plates; in asperula the central and radial 

 plates are conspicuously bigger than the radial shields, but in savignyi this 



is not so. 



There seems no reason for doubting that in its earliest stages the disk- 

 covering of savignyi is identical with that of asperula, and Ludwig (1899) 

 has shown that the same is true of Ophiactis kroyeri. In savignyi, however, 

 the occurrence of schizogonous reproduction in very early life interferes with 

 the orderly and uniform development of the disk-covering, shown by 

 asperula and kroyeri, so that after division has occurred twice it is difficult, 

 if not impossible, to distinguish the primary plates. This is well shown by 

 figure I of plate i, which represents a specimen with a disk rather more 

 than I mm. in diameter. Division has apparently occurred twice and it 

 will be seen how irregular is the resulting disk-covering. The irregularity 

 seems to be due to the fact that whereas, in asperula and kroyeri, the new 

 plates are formed near the distal margins of the radial and interradial 

 plates, in the regenerating savignyi they seem to be formed at the proximal 

 edge of marginal plates. In other words, the typical disk-covering develops 



