He is a young one. About 1.5 inches. Crayfish are crustaceans, just like shrimp and crab.
I caught him in the Conestoga River in the county park. They live under rocks. Usually we catch one every couple minutes but he was the only one we caught all night. He now lives in my fish tank.
When I caught him he only had half of his legs. The current is pretty strong, so I figured it may be a good idea to put him in a safer place. In about two weeks he had regrown all of his legs. He hides in a pirate ship and is constantly rearranging rocks and such. His name is Jimothy.