Mollusks and Crustaceans

• Seashells are usually the hard exoskeletons made by mollusks, but some crustaceans of the arthropod phylum like hermit or lady crabs also produce shells. The shells long outlive the organism that made them and usually end up washed ashore, where they are occasionally used by other organisms that cannot produce their own shells.
  
  

Bivalves

• Bivalves are a common type of mollusk shell actually made of two shells hinged together. Bivalve creatures include clams, mussels, oysters and scallops. These organisms have no head or, depending upon the species, no eyes, and they eat passing plankton that floats by. Shell Beach in Western Australia is almost completely covered by discarded bivalve shells.
  
  

Gastropods

• Gastropod shells from snails have a spiral shell in which the spiral either tucks inside itself or spirals like a staircase up to a point, which is the kind of shell that most people identify when they think of a conch. Slugs are also gastropods but have no shells. Sometimes hermit crabs will wear discarded gastropod shells in order to protect their soft bodies from predators. Hermit crabs have to find different sized shells as they grow older.
  
  

Chitons

• Polyplacophorans like chitons have eight separate plates or valves arranged around their bodies that give them a gauntlet-like appearance. This allows them to roll into balls for protection. When a chiton dies, its girdle no longer holds the shells together, and they separate into smaller parts which are free to float at random. These individual shells are sometimes known as butterfly shells due to their shape.
  
  

Cephalopods

• Cephalopods are the least likely to have shells, but some forms like the nautilus and spirula utilize shells with the familiar spiral shape. Extinct ammonites had the largest shells that to this day sometimes survive as fossils. Cuttlefish have internal shell-like skeletons that sometimes wash up on the beach. Octopuses do not produce shells but are known to use them for protection.