Biologic evolution

I think that the bivalve can classify it into two kinds.

One is the one to put the skin of the transparency on the surface of the shell and to make the shell in that.
The clam is the representative.
Another one is in the shell the skin of the transparency and is the one to make the shell.
The littleneck clam is the representative.
Hard clam is littleneck clam's type.

二枚貝は2種類に分類が出来ると思う。

1つは貝殻の表面に透明の皮を張りその中に貝殻を作るもの。
蛤がその代表です。
もう1つは貝殻に透明の皮がなく貝殻を作るもの。
アサリがその代表です。
ホンビノスガイはアサリのタイプです。