We're here for dinner, not dissection, so we're gonna keep this high level. But a little bit of anatomical know-how is essential for smooth shucking and discerning slurping.
Oysters have two shells - a cupped shell (aka left or bottom shell) and a flat shell (aka right or top shell). The shell shape will tell you a lot about how the oyster was grown. If the bottom shell is extremely round, like a golf ball cut in half, you know the oyster was likely tumbled in some way. If it's less rounded, the oyster was likely beach grown. If it's long and skinny or somehow misshapen, you might have difficulty shucking the oyster.
The two shells are connected together at the hinge, which is located near the pointy end of the oyster, near a conical shaped structure called the umbo, which is also the oldest part of the oyster.
A muscle - the adductor muscle - runs through the oyster connecting the top and bottom shell together. The adductor muscle is the only point of attachment between the oyster and its shell. When you're holding the oyster with the cupped side down and the hinge towards you, the adductor muscle will be on the right side of the shell.
When you shuck an oyster, your goals are two fold: 1) get the knife in the oyster (and it's usually easiest to find the seam between the two shells in the hinge) and 2) cut the oyster free of its shell, by scraping the adductor muscle off the two shells. Check out our step by step shucking instructions here, and watch a video here.
The mantle, or the dark outer ring of the oyster itself, is where the oyster forms its shell. So, don't be dismayed if all your oysters are full of shell - it's not bad shucking, the shell was there to begin with! Before you slurp, look for fragile bits of shell in the mantle, and then use your knife to flick it out.
The gills of the oyster are located near the mantle. Sometimes the gills can pick up color - usually blues and greens - from surrounding algae. These oysters are then called "green gilled" or "blue gilled" oysters.
The belly - not a scientific term - is the wide part of the oyster near the hinge. The belly will tell you a lot about the texture and saltiness of the oyster before you slurp it. Gray and kind of translucent? Prepare for a salt bomb. White and firm? It's gonna be sweet and very flavorful. Veiny and soft? The oyster is a bit spawny, and you might consider cooking it rather than eating it raw.
The fringe is where the new growth appears on the outer shell. Fringes can tell you how the oyster was grown. If the fringe is rounded the oyster was likely tumbled in some way... if it's frilly, the oyster was probably grown on a beach, or at least without a whole lot of agitation.
FROM THE OUTSIDE
In a tumbled oyster, as pictured below, the umbo can sometimes curl up and interfere with hinge shucking. In this case, you just need to find a way to get the knife in the hinge at a steeper angle, or from the side.
The adductor muscle has two parts: a quick, which the oyster uses to open and close its shell, and a catch, which the oyster uses to hold the shell shut. The "quick" is larger and more translucent, the "catch" is smaller, whiter, and crescent-shaped.