ADULT COLONY. 15 



COLOR. 



The polyps on the under, unexposed surface of colonies may be color- 

 less, but vary elsewhere from light to dark brown. Owing to the partial 

 transparency of the tissues the white corallum always shows through, and 

 gives rise to an alternation of light and dark radiating areas and to a light 

 polygonal line separating one polyp from the other. Sometimes the disc 

 presents a velvety green color along the mesenterial radii, and the oral aspect 

 of the tentacles and the angles of the mouth also may be of the same color. 

 The stomodseal walls and knobs of the tentacles are white, while the stems 

 of the latter are usually brown. More rarely the elevated peristome may be 

 rose-colored, and in some cases purple.* 



The different shades of brown exhibited by the polyps are entirely due 

 to the presence of Zooxanthellae or unicellular algae within the endodermal 

 layer. The chromoplasts within these are brown, yellow, or greenish yellow, 

 and when the algae are but few in number they give a light brown color to 

 the walls of the polyps, whereas when present in great numbers, overlying 

 one another, the polyps appear a dark brown. For this reason the thin 

 tissues of fully expanded polyps are much lighter in color than the tissues 

 of retracted polyps. 



Examination of the walls of colorless polyps on the under surface of a 

 colony proves that the Zooxanthellse are either absent or occur very sparingly 

 in the endoderm. When, however, the colorless areas are exposed to light, 

 the polypal tissues begin slowly to assume a brownish color, and in a few 

 days yellow cells are to be found in abundance. 



On examining a living tentacle under the microscope the outer ectoder- 

 mal layer is seen to be quite colorless, but the yellow Zooxanthellse are 

 revealed by focusing through to the internal endodermal layer. The white- 

 ness of the spheroidal tips of the tentacles, compared with the brown color of 

 the stems, is also due to the fact that the knobs are wholly formed of ectoderm 

 the endoderm lining the cavity of the stems not being prolonged into them. 

 The superficial green or purple color is somewhat evanescent, and wholly 

 ectodermal in origin. 



Different shades of yellowish brown are the prevailing colors of the coral 

 areas in Jamaica, and whether in the palmate or branching growths of 



* Le Sueur (1820, p. 286) describes the color of the specimens seen by him at Guadeloupe as " d'un 

 rouge mel de violet." The color of 5. siderra as given by the same author is also very different from 

 that of the Jamaica specimens. Verrill (1901, p. 154) writes thus of the Bermuda 5. radians: "The 

 general color in life is dull gray, yellowish gray, ocher-yellow, or rusty brown, sometimes tinged with a 

 purplish rosy tint; the polyps are paler, with the lips and tips of the tentacles whitish." 



