The word "oyster" has its roots in Greek and is related to Greek words meaning hard shell and bone. The first known use of the word "oyster" was in the 13th century. The true oysters most people associate with those eaten by humans, but there are also the well-known Pearl oysters which aren't closely related to true oysters. Pearl oysters are part of the feather oyster family. There are many other types of oysters as well, or at least animals that have the common name "oyster". Thorny oysters are more closely related to scallops but cement themselves to substrate like true oysters. Saddle oysters are also known as jingle shells. They have a translucent shell which gives rise to their other common name, mermaids' toenails. These oysters aren't eaten as they taste bitter.
Windowpane oysters are somewhat related to saddle oysters with their translucent shell, but they're technically edible. Windowpane oysters don't cement to the substrate and instead live in mud. Their shells have been used as a glass substitute in glazing. At the same time, there are true oysters who look completely dissimilar to their true oyster relatives. While they still cement themselves to substrate, this genus of true oysters have heavily ribbed shells with uneven zigzag patterns. Those aren't the only oddly shaped true oysters alive today.
The fronded oyster gets its common name from its wavy leaf-like appearance. This just goes to show that looks aren't everything when it comes to animal relationships. Even ancient oysters came in some strange shapes. This c-shaped extinct true oyster genus is believed to have lived during the late Cretaceous, which was right up to when most dinosaurs went extinct. The oyster genus most commonly used as food for human consumption has this name. The name means "thick oyster", which means people tend to like the thick-shelled, large oysters for food. The phrase "the world is your oyster" comes from Shakespeare's "The Merry Wives of Windsor". However, it's a misquote. In the comedy, the original line is "The world's mine oyster, which I with sword will open". You've also probably heard that oysters are asymmetrical and can handle changes in water salinity.
