Shellfish are distinguished from fin fish by their hard outer shells and their lack of backbones or internal skeletons.
There are two classifications of shellfish:
1. Mollusks are soft sea animals that fall into three main categories:
• Bivalves,which have a pair of hinged shells (such as clams and oysters).
• Univalves, which have a single shell (such as abalone and conch).
• Cephalopods (such as octopus, squid, and cuttlefish).
From the scientist's point of view, there are other mollusks as well, but they do not concern us here.
2. Crustaceans are animals with segmented shells and jointed legs.
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