166 



THE RATE OF GROWTH 



[CH. 



The window-pane oyster in Ceylon (Placuna placenta) has been 

 kept under observation for eight years, during which it grows from 

 two inches long to six (Fig. 36). The young grow quickly, and slow 

 down asymptotically towards the end; an S-shaped beginning to the 

 growth-curve has not been seen, but would probably be found in 

 the growth of the first year. Changes of shape as growth goes on 

 are hard to see in this and other shells ; rather is it characteristic oi 



Fig. 36. 



12 3 4 5 6 7 



Age in years 



Growth of the window-pane oyster; short diameter of the shell. 

 From Pearson's data. 



them to keep their shape from first to last unchanged. Nevertheless, 

 shght changes are there; in the window-pane oyster the shell grows 

 somewhat rounder; in seven or eight years the one diameter multi- 

 plies (roughly speaking) by eleven, and the other by ten*. 



Window-pane oysters (Placuna) 



Ratio 



117 

 109 

 107 

 1-06 

 105 



The American slipper-limpet has lately and quickly become a pest 

 on English oyster-beds. Its mode of growth is interesting, though 



* Joseph Pearson, The growth-rate. . .oi Placuna placenta, Ceylon Bulletin, 1928. 



