76 PROCEEDINGS OF THE NATIONAL MUSEUM vol. loi 



Not until a few years ago, however, was the nature of this change 

 demonstrated. Dr.Hitchcock (1941, pp. 26-30) found that the chroma- 

 tophores of specimens of P. minutus he investigated at Bermuda were 

 responsive to white, black, red, blue, yellow, and green back- 

 grounds, but that there was no immediate change in the general color 

 of the crab because of the extracellular pigment in the hypodermis and 

 exoskeleton. These findings seem to indicate that the apparent color 

 of any particular specimen can be changed only at the time of molting, 

 and any individual that moves to a differently colored background 

 subsequent to hardening of the shell would cease to be protectively 

 colored. Interesting results might be obtained by confining molting 

 specimens to backgrounds of various colors and patterns. 



The color of P. cyaneus has been recorded as cerulean blue, bluish 

 gray mottled with brown, yellowish clouded with brown, and reddish 

 brown. It is probable that a type of color change comparable to that 

 in P. minutus occurs in this species as well, although no similar experi- 

 ments on it have been attempted. 



• Plones minutus 

 o Plones cyaneu* 

 + Pochygrapsus morinus 



8 9 10 12 



Coropace Length (mm.) 



Figure 5. — Relative growth of the carapace in Planes minutus. Planes cyaneus, and Pachy- 

 grapsus marinus, based on mean values (table 2) plotted logarithmically. 



