Growth-changes in Brittle Stars. ill 



8. The next to the lowest arm-spine and not the lowest is the first one 



formed, but the first segment usually has both, on each side, before 

 the second segment arises. 



9. The lowest arm-spine never develops any special or characteristic 



form. 

 10. The distal arm-segments have no tentacle-scales, but at the base of 

 the arm each pore has two; one, associated with the side arm- 

 plate, arises after 12 to 18 segments have been formed; the other, 

 associated with the under arm-plate, arises one or two segments 

 later. The former may be homologous with an arm-spine; the 

 latter may be homologous with the tentacle-scale of Ophiactis. 



Ophiothrix axgulata (Say). 



Two species of Ophiothrix, angulata and cerstedii, are very common in 

 shallow water throughout the West Indian region. Both are exceedingly 

 variable in coloration and the specific differences between them are often 

 hard to make out. The commoner and more variable species is angulata, 

 and at Montego Bay a dozen specimens could be gathered to every one of 

 cerstedii. The latter has very characteristic lines of white or cream-color 

 cross-banding the upper surface of the arms, and these are rarely, if ever, 

 lacking, though they are occasionally faint. In angulata such cross-lines 

 are rarely indicated, but there is present a longitudinal stripe of white, or 

 some light shade, running the length of the arm. Although such a dis- 

 tinction seems trivial, especially when the rest of the coloration is very 

 variable, it is really remarkably constant and specimens can generally be 

 assigned to one species or the other at a glance. 



As is generally the case when a species is very common, angulata occurs 

 in very diverse habitats. It is quite common at Montego Bay among 

 coral fragments on the reefs, it is very common among the eel-grass on the 

 flats, and it is exceedingly common among the bryozoa, sponges, mollusks, 

 hydroids, and ascidians growing in masses on the mangrove roots among 

 the Bogue Islands. Most of the young specimens obtained were found in 

 the last-named habitat, especially in the cavities of the same red sponge in 

 which Ophiactis and Amphipholis occur. No very young specimens were 

 found, the smallest having the disk 2 mm. across. While this is greatly to 

 be regretted, much interesting information has been secured from the young 

 ones that were obtained. No indication of schizogony nor of variation 

 from the typical pentamerous form was seen. 



COLORATION. 



Neither Ophiactis nor Amphipholis has a color pattern sufficiently 

 distinctive to make its development a matter of any significance, but as 

 already stated, Ophiothrix angulata is most easily distinguished from its 

 nearest ally by a characteristic color pattern. The development of this 

 pattern, therefore, is of no little interest. The youngest joints at the tip 

 of the arm are colorless and contain no pigment. In the third, fourth, or 

 fifth segment, however, a little pigment begins to appear and gives a tinge 



