148 MARVELS OF FISH LIFE 



Examine the spat under a microscope as it escapes 

 from an oyster. Each embryo is seen to be rushing 

 about, helter-skelter, in all directions. Presently you 

 will detect one lying still, and if lying flat you will see 

 only one of the delicate translucent valves ; if lying 

 end up, you will see both the valves, which at this time 

 are shaped alike. As you watch one of the stationary 

 spat, a pad-like structure, known as the velum, will 

 protrude between the valves. This velum is covered 

 with cilia (the hair-like filaments already described), 

 and these ^moving rapidly drive the spat through the 

 water. Watch carefully and you will see a spat come 

 to rest, for as it swims the velum is suddenly pulled in, 

 and the valves closing over it, the spat stops with a 

 jerk. 



Each oyster expels about three million spat. Imagine 

 the number shed over a large oyster bed ! 



If within forty-eight hours the spat finds a suitable 

 resting place to which it can become attached, it may 

 ultimately grace a Lord Mayor's banquet. But should 

 it be carried about in the sea by strong tides and rough 

 weather for longer than this period, the spat will perish. 



Now, let us study the life of an oyster on a farm. 

 First the beds are thoroughly cleaned of all seaweed, 

 and the " hassack banks " are, as far as possible, des- 

 troyed. Hassack banks is the name given to the mounds 

 of mud and debris surrounding colonies of tube-building 

 worms. On the clean oyster bed are put down quantities 

 of bleached oyster shells, technically known as " cultch." 



