ii8 Papers from the Marine Biological Laboratory at Tortugas. 



from the arm-tip. It has one notable peculiarity in that foramina are 

 beginning to open through it. Examination of the lowest spine of segment 

 nine, counting from the mouth, shows (pi. 3, fig. 8) a still further reduction 

 of teeth, particularly the terminal one, while there is a corresponding 

 development of the foramina. The lowest spines of the segments still 

 nearer the mouth (pi. 3, figs. 9 and lo), show these tendencies carried still 

 further. Indeed, except for their smaller size, these basal lowest spines are 

 not essentially different from the ordinary spines of the arm. 



The development of the other arm-spines offers no such interesting 

 features as those shown by the lowest. The next to the lowest is the second 

 one to appear and is generally present on the second segment. The others 

 appear in regularly vertical sequence dorsally. The third spine is to be 

 found on the fourth segment from the tip and the fourth on the ninth. It is 

 not until about 40 segments are developed that a fifth spine appears, while 

 the arm has no less than 60 joints when the sixth spine is to be seen. A 

 seventh spine will arise if about 75 joints are formed, and an eighth if the 

 number is about 90. I have not found more than 8 spines in any specimen, 

 and think that 6 or 7 is the characteristic number for the species. 



The tentacle-scales are reduced to very narrow, sharp spinelets,, which 

 are quite lacking at the tip of the arm. The first indication of them is on 

 the sixth or seventh segment, where a minute one is to be found on each 

 side, connected with the lowest part of the side arm-plate. They never 

 reach a large size, but even on the basal segments give the impression of 

 being functionless remnants. There is no indication whatever of homology 

 with an arm-spine. 



SUMMARY OF GROWTH CHANGES IN OPHIOTHRIX ANGULATA. 



1. Although the coloration is distinctive, it is acquired very early without 



passing through any recognizable stages. It is due to the deposit 

 of purple pigment in the arm-segments longitudinally and the 

 usually complete failure of any pigment to cross the median line 

 either dorsally or ventrally. When the color is other than some 

 shade of purple, it is due to the masking of the purple pigment 

 by a secondarily added color. 



2. The first-formed podia are smooth, as is usual in ophiurans, and the 



papillose condition characteristic of the genus Ophiothrix is only 

 acquired after about 10 segments are complete. 



3. The central plate of the youngest specimens examined is a dispro- 



portionately large, homogeneous, thin disk with a few minute per- 

 forations around the margin. During the growth of the disk this 

 plate is gradually resorbed, becoming not only much smaller 

 (actually as well as relatively) but more and more reticulate from 

 the increasingly numerous foramina. 



4. The disk-plates carry trifid spinelets, which not only become more 



numerous with growth of the disk, but have a tendency to elongate 

 into slender spines. The number of these disk-spines is very much 

 subject to individual diversity. 



5. There are no oral papillse in Ophiothrix, but after 4 or 5 teeth are 



formed, dental papillae (homologous with the teeth, but much 



