24 



THE ROYAL SOCIETY OF CANADA 



ment of the "light" box, and two hydroids {Bougainvillia and Tuhu- 

 laria) in that of the "dark" box. Small mussels from 7 to 8 mm. in 

 length were obtained from the fronds of rockweed (Ascophyllum) 

 growing in the intertidal zone near a large mussel bed. About 

 twenty of these were placed on galvanized wire screening in the 

 central compartment of each box. 



After 24 days the mussels were taken out, measured, and re- 

 placed. After another period of 19 days they were again taken out, 

 measured, and then preserved in formaldehyde solution. 



Even before they were measured it was quite evident that the 

 mussels in the dark had grown much more rapidly than those in the 

 light. There was also noticeable some difference in shape, the former 

 being somewhat thinner (see Figure 2). The results of the measure- 

 ments are given in averages in the accompanying graph (see Figure 3). 

 It is seen that the increase in length was over three times as great in 

 the dark as in the light. 



Fig 2 

 Mussels at end of experiment; dorsal view, X2| 

 A — from lighted box. B — from darkened box 



The difference in shape between the two lots suggested the 

 possibility that the growth of the mussels was not correctly indicated 

 by the changes in length. Three mussels were taken from each lot 

 for the purpose of making a detailed study of the matter. The 

 average ratio of depth to length was found to be identical for the two 

 lots, namely 0.55, but the average ratio of the thickness to the length 

 was greater in those from the "light" box (0.46) than in those from 

 the "dark" box (0.42). 



An attempt was then made to discover whether the increase in 

 weight had been greater in those that had been exposed to light than 

 was indicated by their dimensions. Considering the volume to be 

 the product of the three dimensions in mm. multiplied by a constant 



