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Common Seashells

If you've ever taken a vacation to the beach, chances are good that you have spotted at least a couple of seashells. Seashells come in a wide variety of shapes, colors and sizes. They serve as the protective covering for many types of mollusks, which include clams or other creatures with soft bodies. Many people collect seashells for their beauty or their rarity. There are thousands of types of seashells, but the most common fall into one of three categories: gastropods, pelecypods or cephalopods.

Gastropods
- Snails, which are a type of gastropod, have a single, coiled shell.
  
  Gastropods also are called univalves. Animals classified as gastropods include snails, abalones and conchs. Most gastropods, but not all, are protected by a single coiled shell. They can live in diverse habitats, and they will survive for as long as 30 years.
Pelecypods
- Clams, a type of pelecypod, are covered by two shells that are connected by a hinge.
  
  Pelecypods, or bivalves, are easily identified because these mollusks have two shells connected by a hinge. Bivalve shells provide a protective exterior for clams, scallops, oysters and mussels. Bivalves generally are not very active creatures. They tend to live in mud or sand near the ocean or near fresh water.
Cephalopods
- The nautilus is the only type of cephalopod with an exterior shell.
  
  Even though there are different kinds of cephalopods, only the nautilus produces an exterior shell. Octopus and squid are other examples of cephalopods. These types of creatures are very active, moving quickly from one place to another. They have big eyes, strong jaws and appendages called tentacles, which they use to grab other creatures so they can feed themselves.
Other Types
- Additional types of shells include chiton shells, such as the one at lower right, which have eight overlapping plates.
  
  Chitons and tusk shells are two other main types of seashells. Some chitons resemble worms, and most are covered with a shell consisting of eight overlapping plates connected by a muscular band. They generally live on rocks near the seashore, eating algae. Tusk shells, also known as scaphopods, are shaped like an elephant's tusk. These shells provide protection for a type of mollusk that doesn't have gills, a head or eyes.

Nature