[huntsman] 



EFFECT OF LIGHT ON MUSSEL 



27 



the amount of the average increase in mass of the specimens kept in 

 the dark was 3.11 times as great as that of those exposed to light. 



"Dark" specimens. 

 "Light" specimens. 

 Original specimens. 



Weight to volume 

 (aver, of shell and body) 



1.54 X 

 1 X 

 1.27 X 



( 



Note. — X is a constant. 



14.43X0.42 7.5^X0.44 



1.54 X 



1.27X 



) 



( 



9.53 X 0.46 7.5^X0 



1 X 



1.27X 



z~ ) 



= 3.11 



It may be asked what the significance is of the differences between 

 the two lots in regard to shape and relation of weight or mass to 

 volume. The light rays have doubtless hindered growth by injuring 

 some or all of the tissues of the mussel. The shell of the mussel is 

 quite opaque as it contains a large amount of black pigment. Indeed 

 it was expected that owing to this protective coat light would be 

 found not to have any effect upon the mussel's growth. The shell 

 increases in length and depth by additions to the free margin, and 

 it is there that the living material is unprotected by shell or only 

 imperfectly protected by the thin growing edge. As it has been 

 found that the light retarded growth chiefly in respect to increase in 

 length and depth, it is fairly certain that the greatest injury has been 

 done to the more or less exposed growing edge of the mantle at the 

 margin of the shell. For this reason length and depth are not satis- 

 factory criteria of the rate of growth when differences of lighting 

 occur. 



There were definite indications of the importance of light in 

 developing the full intensity of the black pigment in the shell. The 

 newly formed shell in the specimens kept in the dark was almost 

 invariably of a more or less light brown colour, in striking contrast 

 to the very black original shell. In the other lot the contrast was 

 slight or not at all evident. 



There are very few accounts in the literature of the effect of 

 light upon the growth of animals. Higginbottom (1850, p. 435) 

 found that the tadpole of Rana temporaria "advances in growth 

 equally well in the dark and in the light." Torrey and Martin (1910) 



