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.” 
