One of my fondest memories is sitting on a riverbank in Portugal shelling and eating freshly steamed fresh shrimp.
Crustaceans are shellfish that have an external skeleton.
They all have legs and are able to move. Some, like lobsters, have a jointed shell that helps them to move. Crabs do not have a jointed shell but use their legs to scurry about. Crustaceans are members of the Arthropoda family that includes spiders, scorpions and insects. They are covered in a hard horny shell or, more correctly, carapace.