DESCRIPTION 29 



unpub.). H. L. Clark (1933, p. 80) found a difference in color in 

 northern and southern forms. "While northern specimens have the 

 test commonly a deep brown, with a reddish cast, and the spines 

 lighter and often a dull brownish red, in the south there are two ex- 

 tremes; on the one hand, the test is a light wood-brown, with spines 

 very light (a dingy cream color) at the base, becoming dull reddish 

 purple at the tip, and on the other, test and spines are deep reddish 

 purple or almost black in certain individuals. Both these extremes 

 are observed in specimens from Florida. Specimens from Cuba, Yu- 

 catan, and Tobago, are very dark and have slender, relatively long, 

 spines." 



The color change in /l. lixula was studied many years ago in Naples 

 by von Uexkiill (1896), who found that they became black in the light 

 and brown in the dark. I confirmed this in Naples in 1934 (unpub.) 

 finding that light specimens of yl. lixula became darker in the light and 

 dark specimens became lighter in the dark over a period of a month; 

 in some individuals which were isolated, the change was noticeable 

 within a day. Kleinholz (1938) found that from a group of 12 dark 

 specimens of^ A. lixula at Naples the ones kept in the light for 6 hours 

 remained dark, while those kept in the dark became brownish. Some 

 dark individuals turned brownish in the dark in 90 minutes, and when 

 brought into the sunlight became dark again in an hour. The two 

 species of Arbacia react to light by change in color in much the same 

 way, but A. lixula reacts more rapidly. In nature A, lixula is much darker 

 than A. punctulata. 



Centrostephanus longispinus (at Naples) reacts in much the same way, 

 a dark animal turning gray in the dark within two hours (von Uexkiill, 

 1896; Kleinholz, 1938). Von Uexkiill found that the change in body 

 color was due to changes in the chromatophores of the body skin which 

 were expanded in the light and concentrated to small points in the 

 dark. Kleinholz found that the tube feet isolated from an illuminated 

 animal were brownish red while those from an animal kept in the 

 dark were pinkish white, owing to the dispersed or contracted phase 

 of the chromatophores. 



A still more rapid change in color occurs in Diadema (Centrechinus) 

 antillarum in Jamaica, especially in the young animals; they become 

 lighter, almost white in the dark (Millot, 1950). The change is due to 

 changes in the large skin chromatophores, expanded in the light and 

 contracted in the dark. Even a spot of light will cause the change. 



No albino specimens of Arbacia have been reported, but Prince 

 (19 1 3) has described an albino of Strongylocentrotus drbbachiensis from 



