112 Papers from the Marine Biological Laboratory at Tortugas. 



of color, deepening with the increasing age of each successive segment. 

 This pigment is bright purple of a deep shade. It first appears in the side 

 arm-plates near their upper ends and gradually spreads distally and to 

 some extent ventrally. It soon invades the margins of the upper arm- 

 plates and follows them to their distal end. In a typical case it is very 

 sharply confined to the sides of the upper arm-plates and fails to cross from 

 one side arm-plate to its fellow of the same segment. Consequently there 

 is an unbroken, unpigmented (i. e., apparently white) stripe along the dorsal 

 surface of the arm. In some specimens pigment fails to invade the distal 

 corners of the upper arm-plates and thus there appears to be a row of white 

 spots along each side of the stripe; or the pigment may fail in the spine- 

 bearing ridge of the side arm-plates and then the dorsal white area of the 

 arms is markedly increased. On the lower side of the arms the pigment 

 also spreads longitudinally in the side arm-plates and in the margins of the 

 lower arm-plates, but it rarely if ever crosses the latter. In other specimens 

 the pigment spreading in the margin of the upper arm-plates follows back 

 along the converging sides to the proximal end and there is confluent with 

 its fellow of the other side and with the pigment of the side arm-plates. 

 In such cases the longitudinal white line is broken into a series of spots, 

 formed by the mid-anterior and central parts of the upper arm-plates. 

 When pigment is formed very abundantly the white stripe, both dorsally 

 and ventrally, is very narrow and the general coloration is uniform deep 

 purple. Many specimens, however, are not at all deep purple, but are 

 very light colored, or are brownish, reddish, or greenish. This diversity of 

 color is very easily explained. Light-colored specimens are simply those 

 which have developed little pigment. Green specimens are those in which 

 the purple pigment is masked by a green coloring-matter acquired after 

 the formation of the purple; whether this green matter is another pigment 

 or consists of symbiotic algse I am not prepared to state, as no study of the 

 point was made on living animals. Brown and red shades are produced by 

 the masking of purple by other substances. In all cases, however, purple 

 pigment is the primitive coloring-matter. 



The coloration of Ophiothrix cerstedii begins, like that of angulata, by 

 the development of pigment in the upper part of the side arm-plates. But 

 the pigment spreads laterally and not longitudinally and the whole develop- 

 ment of the color-pattern is thus entirely different from that of angulata 

 and there is never, and naturally there can not be, an uninterrupted white 

 stripe either above or below. The first bit of pigment to appear may 

 generally be seen in the third or fourth arm-segment in the upper part of 

 the adoral (proximal) portion of the side arm-plate. This rapidly spreads 

 across to meet its fellow from the opposite side and also descends along the 

 side arm-plate ventrally, where it also ultimately meets its fellow from the 

 opposite side and thus completes a narrow purple ring around the arm. 

 Meanwhile another patch of pigment arises just above the upper arm-spine 

 on the side arm-plate and spreads on to the upper arm-plate and down 



