Mollusks:
* Snails: These gastropods have a single spiral shell.
* Slugs: Some slugs still have a small, internal shell, while others have completely lost it.
* Clams, oysters, mussels, scallops: These bivalves have two hinged shells.
* Nautilus: These cephalopods have a spiral, chambered shell.
* Squid and Octopus: These cephalopods have an internal shell, often reduced or absent.
Arthropods:
* Crabs, lobsters, crayfish, shrimp: These crustaceans have a hard exoskeleton made of chitin.
* Turtles: These reptiles have a bony shell that protects their vital organs.
Other:
* Tortoises: These land-dwelling turtles have a shell that is dome-shaped for protection.
* Chitons: These marine mollusks have eight overlapping plates that form a shell.
* Sea urchins and sand dollars: These echinoderms have a hard, spiny shell.
Note: Not all animals with shells have them for the same reason. Some shells are for protection, others are for structural support, and some are for buoyancy.