IV] LIFE-HISTORY AND ENVIRONMENT 43 



for our edible mussel. It does not hold good, how- 

 ever, for the fresh-water mussels of our streams and 

 lakes, the life-history of which will be discussed 

 further on. The small egg, at first irregular in 

 shape, but becoming spherical after fertilisation, 

 floats in the sea at the mercy of any current over 

 the banks ; but development continues, and we soon 

 have a little multicellular embryo which swims by 

 means of cilia. This power of locomotion is of course 

 of no account in determining the wanderings of the 

 embryo, which, during its entire life, is dependent 

 upon the erratic currents in which it floats. The 

 second day marks the appearance of a tiny shell 

 which develops until a minute transparent bivalve 

 shell encloses the larval oyster. It is still a swimming 

 form living as a floating organism. This brings us 

 to the stage immediately before the formation of 

 "spat." The term "spat" is given to the earliest 

 attached stage in the life-history of molluscs like 

 the pearl oyster. The attached stage is probably 

 reached just within a week after fertilisation has 

 taken place. 



It will be seen that the chances are very great 

 that during this time the free swimming embryo has 

 been carried by tidal currents far away from the 

 home of its parent oysters. It may perhaps be taken 

 over a great oceanic abyss, or a sea-floor quite 

 unsuitable for its attachment, in which case the adult 



