Biologic evolution

The fisherman pouches the corbicula and it brings it.
Because the corbicula is in the state of just going up from the river,
the sand of the river is attached to the shell.
Water is put on the bucket, and the corbicula is put in that.
The sand that is attached to the shell drops when the corbicula is
washed in water.
The one that the dropping sand was collected was put in the PET
bottle.
The amount of the sand put in the PET bottle has decreased it from
the right of the photograph.
It is the same that sand is the least in a right edge ..there is a
lot of sand.. and the amount of water ..left end.. to have is same.
The color of water changes if water mixes with sand.
When the amount of sand is a little, it is brown.
When the amount of sand was large, it became black.