104 Papers from the Marine Biological Laboratory at Tortiigas. 



to the third, arises very much later; in some individuals it may be found on 

 the fifteenth segment back of the terminal plate, but in other specimens as 

 many as 25 segments may have only 3 spines on each side. In large speci- 

 mens, a fifth, sixth, seventh, and very rarely an eighth spine appear in each 

 series dorsal to the fourth, but the position of the segments on which these 

 spines first appear difters much in different specimens. In the largest 

 specimen seen, the basal arm-segment has 2 spines, the next 4, the next 5, 

 the next 6, the next 7, and the next 8; then follow 13 joints, each with 7 arm- 

 spines on each side, 23 joints with 6, 24 with 5, and 18 with 4; the rest of 

 the arm is unfortunately missing. The species character (5 or 6 arm-spines) 

 is thus shown from the tenth to the fifty-seventh segment. This is a much 

 more extended series than is shown by the vertebrae, which are typical from 

 about the eighth to the twentieth segment only. The next to the lowest 

 arm-spine is the largest (longest and stoutest), as long as there are only 4, 

 but as the number increases the lower spines grow very little, while the upper 

 ones grow rapidly, so that near the base of the arm the next to the upper- 

 most, or the uppermost, becomes the largest. At the tip of the arm, the 

 spines are smooth, but they soon develop thorn-like projections on the 

 lower, adoral margin, and generally on all sides, especially near the tip. 

 Excepting on the lowest spine, these projections are neither large enough nor 

 sufficiently definitely arranged to have any particular significance, but on 

 the lowest spine they are more characteristic. The lowest spines of the 

 third segment (pi. 3, fig. 12) have an enlarged base and a smooth, nearly 

 cylindrical tip, but those of the sixth (pi. 3, fig. 13) have the thorn-like pro- 

 jections or "teeth" of the lower, adoral margin very conspicuous, especially 

 the terminal one. There are half a dozen or more of these teeth at this 

 stage, but as the spine grows older they increase in number and occupy the 

 other angles of the spine, so that at the middle of the arm the lowest spine 

 (pi. 3, fig. 14), although much more thorny than any of the others, does not 

 differ from them as evidently as it does near the tip of the arm. It can not 

 be said, then, that the lowest arm-spine of Ophiactis is, in any sense, char- 

 acteristic. 



The tentacle-scales in Ophiactis savignyi are large and conspicuous, 

 although there is but one on each pore. It arises almost as soon as the 

 lowest arm-spine, but it is associated with the ventral surface of the side 

 arm-plate. It arises from a separate center of calcification and its rudiment 

 appears simultaneously with that of the under arm-plate. There is nothing 

 in the development of the tentacle-scales to determine whether they are 

 homologous with the arm-spines or not. 



SUMMARY OF GROWTH CHANGES. 



1. Fission occurs as soon as adult form is assumed; it usually results in 



complemental halves, but may leave /owr rays on one half and only 

 two on the other. 



2. The 5-rayed individuals arise in either one of two ways: by the forma- 



tion of a normal 3-rayed half on a regenerating 2-rayed individual. 



