STUDIES ON SEX IN CREPIDULA 5 



and in the older, crowded ones; for as the swimming veliger 

 settles and commences its growth, crawling about to secure a 

 favorable location, it has a smooth, uninterrupted, slightly con- 

 cave surface in the 'new' shell on which to rest; while in the 

 already thickly populated 'old' shell it must adjust its growth 

 over the shell of some large individual already in possession, or 

 between two of them, or behind them. If it moves it will find 

 itself, at the end of each small journey, on a new sort of sur- 

 face, to which it must again adjust itself if it remains there long 

 enough for the mantle to be extended and the shell substance 

 secreted. In the 'new' hermit shells, inhabited by only a few 

 Crepidulas which do not interfere with one another, those 

 Crepidulas have thin, smooth, flat shells with few lines of 

 growth or none; while in the crowded colonies the shells are 

 more convex, thicker, and marked with numerous lines of growth, 

 each of which indicates a movement to a new position, a growth 

 over an irregular surface, or some other hindrance to the con- 

 tinuous free extension of the mantle. 



Frequency of movement is a factor which affects the shell of 

 Crepidula. It can be observed, in large colonies, that indi- 

 viduals close to the mouth of the hermit's shell do not often 

 change their position, and their shells become intimately fitted 

 to the surroundings. There is more shifting about among the 

 individuals behind them in the deeper recesses of the hermit's 

 shell; it is quite probable that these latter have a less advan- 

 tageous position for securing food. The edges of their shells 

 are smooth and rounded, not fitted to the underlying surface. 



Conditions are different in 'new' hermit shells. When a 

 swimming veliger of C. plana settles there, it finds a smooth 

 uninterrupted surface; it quickly reaches an advantageous po- 

 sition and henceforth seldom moves. A thin, smooth shell, 

 with almost indistinguishable growth marks, is evenly deposited 

 and closely applied over the substratum. 



C. Eyivironmental polymorphism 



Conkhn has described the condition of 'environmental di- 

 morphism' in C. plana. In the dead shells of Ilyonassa or Lit- 



