No. 634] 



BIONOMICS OF MELLITA 



441 



that the exposed posterior or postero-lateral margin projects in a 

 not inconspicuous way above the general level of the sand. 



In the light of this behavior, and particularly because of the 

 injuries found to have been suffered by the sea-plates, it be- 

 comes pertinent to inquire whether an adaptive (concealing) 

 value attaches to the pigmentation of these animals. The oc- 

 currence of injured specimens, and that in some degree of fre- 

 quency (about 60 per cent, of those above 9 cm. transverse diam- 

 eter), would seem in itself to be valuable evidence upon this 

 point. There are several other important considerations to be 

 derived from the nature of the pigmentation of Mellita. 



Until it has attained a diameter of 7 to 8 cm., the young M. 

 sexies-perforata, seen from above, is practically colorless; the 

 integument contains no pigment, 

 although the yellow-brown stomach /^H^\ 

 may show faintly through the test, \ >v 



Upon attaining this size, a light coffee- 

 tint, evenly distributed upon the 

 dorsum, makes its appearance ; previ- 

 ously, at about 5.7 cm. diameter, dark 

 brown pigment begins to show on the 

 ventral surface, on each of the poly- 

 gonal areas surrounded by the tube- 

 foot channels. Pigment thus begins fig. 6. illustrating natural 

 to be deposited on the ventral side ; 1 ° J i " ry From in the a ii^n ge an°imar 

 and it continues to be denser (darker) (x F ° ° 

 on this side than dorsally. The in- 

 tensity of pigmentation increases progressively with age, until, 

 in animals on 12-13 cm. diameter, a very dark brown hue is 

 attained. 



M. sexies-perforata, at Bermuda, does not frequent bottoms 

 supporting a good growth of eel-grass. If in other regions it 

 should be found to do so, the alkali-greening substance occur- 

 ring in this species (Crozier, 1918) and in Clypeastroids gen- 

 erally, might be adaptively concerned in pigmentation. But no 

 green hues are normally evidenced by this species at Bermuda. 

 If the somewhat uncertain records of a green coloration in nor- 

 mal mellitas of this species at more southerly stations are con- 

 firmed, and found related to an eel-grass habitat, a direct phys- 

 ical explanation is at hand to account for the greening (cf. Cro- 

 zier, 1918). The normal brown hue seems due to the integu- 



