A STUDY OF METAMERISM. 439 



time, and when we recall the complicated modifications of the 

 terminal segments, it seems highly probable that posterior to 

 the compound metamere no shifting of the segments of one 

 side can have taken place, and that no one segment has divided, 

 but that the plate as a whole has broken up into a greater 

 number of half-metameres on one side than on the other. 



XI. Study of the Colour-bands of Echinoderms. 



In the summer of 1891, while in Jamaica at the Marine 

 Laboratory of the Johns Hopkins University, I began a study 

 of the colour-bands on the arms of the Ophiurians in the hope 

 of getting results that might help towards a solution of the 

 problem of metamerism. The work was laid aside for two 

 years, during which time the colour was so far removed from 

 the alcoholic specimens as to render renewed study unprofit- 

 able. My earlier results, although not carried very far, point 

 to certain conclusions that are not without interest in the 

 present connection. In several species of Ophiurians {" brittle- 

 stars^') the arms are banded at regular intervals by pigment 

 of a different colour from that of the rest of the arm. Each 

 baud of pigment is confined to a single segment — metamere — 

 of the arm. In fig. 96 three such pigmented segments are 

 shown. The colour of the band in this case is a rich orange- 

 red, while the rest of the arm is an olive-green. Along the 

 mid-ventral line there runs a longitudinal narrow line of 

 orange-red pigment not shown in the figure. 



We find that a definite number of uncoloured segments 

 alternate with a coloured segment. Three uncoloured seg- 

 ments lie between the coloured segments, i. e. every fourth 

 segment is coloured. Occasionally, however, the regularity of 

 the arrangement is disturbed. 



Fig. 97 shows a common variation. Instead of finding a 

 fourth coloured segment between the upper and lower coloured 

 segments of the figure, we find two consecutive segments 

 coloured, each over half its extent. The middle line of the 

 arm marks the extent to which each is pigmented, the first 

 half-band lying to the right and the more distal to the left. 



VOL. 37, PART 4. NEW SER. G Q 



