Shell and devein shrimp
Shelling shrimp yourself is stupidly easy and it can usually save you around a dollar or two per pound. The only necessity I can think of is a really sharp small utility knife. A dull knife will make this job alot harder than it needs to be.
The interesting factoid about shelling shrimp is that the "vein" ain't a vein at all. It's an artfully named euphamism for the digestive system. The vein is pretty much the colon of the shrimp.
Procedure
Hold the shrimp between your thumb and forefinger with the tail end extending away from your thumb. Slide the tip of your knife along the apex of the back of the shrimp. Cut through the shellby pushing the knife forward while angling the heel of the knife up slightly. It's almost exactly the same motion you'd use to open an envelope with a knife.
Once you have sliced all the way to the tail, set the knife down and pull the shell off by prying it apart where you made the cut. It will pretty much come off in one motion. If you need to clean up any extra shell and any legs that might remain.
Style note: You can leave the very last section of shell including the tail. The tai is left on usually if you are going to serve shrimp as a finger food so your guests have someplace to grap. If your shrimp is going into another dish like pasta or gumbo it's perfectly accetable to remove the entire shell--tail and all.
Once you have the shell removed it's time to remove the vein. You'll notice that a slice should be running along the back of the shrimp where you cut the shell. In this slice you should see a tiny, clear membrane. It may contain some colored substance (usaull brownish or orangish). This is the vein. Use the tip ofyour knife to grab it and pull gently to dislodge it. It should pull free very easily.
That's it. Yummy shrimp deveined and you saved a couple of bucks!
