Mollusca is the biggest phylum of marine animals, making up nearly a quarter of all marine life. A highly diverse phylum, mollusks vary massively in their biological characteristics and natural environments, ranging across land, river and sea. Mollusks have a long evolutionary history, possibly stretching back to the Cambrian period more than 500 million years ago. Their products, such as pearls, sea silk and Tyrian purple dye, have been considered luxuries in different cultures for centuries. The phylum of Mollusca is usually divided up into ten classes of animal, two of which are extinct. The following are some of the most notable examples of this amazing group of animals.
The Class Gastropoda
Gastropods are most commonly known as slugs and snails. Class Gastropoda is only behind the insects in the number and variety of its species, and examples of the class live on land and in fresh and salt water. The individual biology of species within the class varies enormously. Snails are differentiated from slugs by their large external shells, which they can completely withdraw into when necessary.
Gastropods, Slugs and Snails: A wealth of information on Class Gastropoda, with a large image gallery.
The Gastropoda: An article on gastropods and their evolutionary history, with a gallery of the many forms their shells take.
The Characteristics of Gastropods: An article detailing the nature of Class Gastropoda, and its subclasses.
Introduction to Gastropoda: An overview of gastropods, their life-cycles and feeding habits.
Key for Gastropoda with Shells: A guide to identifying gastropods by their shell characteristics.
The Class Polyplacophora
Members of Class Polyplacophora are more commonly referred to as chitons. They are a marine mollusk with shells made of eight separate plates, which are flexible enough to allow locomotion and also for the mollusk to curl up into a defensive ball.
Taxonomic Heirarchy of Polyplacophora: An in-depth guide to Class Polyplacophora.
Class Polyplacophora and its Characteristics: An overview of chitons, detailing their biology, life-cycle and environment.
Polyplacophora Gallery: Images of common members of Class Polyplacophora.
Polyplacophora Shell Classes: An illustrated guide to different species of chitons.
The Class Bivalvia
Class Bivalvia, or bivalves, is a class of freshwater and marine mollusk. They are filter feeders, and some species attach themselves to surfaces, while others dig down into sediments. Their defining characteristic are shells composed of two mirror-image halves or valves, hence their taxonomic name. Common bivalves include clams, mussels and oysters.
Introduction to Bivalvia: An overview of Class Bivalvia, with images.
Bivalvia Gallery: A photographic collection featuring the varied shells of Class Bivalvia.
Bivalves: A resource on Class Bivalvia, with links to articles and databases of information.
The Class Cephalopoda
Cephalopods are mollusks who have evolved the mollusk foot into arms or tentacles. They possess prominent heads and are bilaterally symmetrical. Some cephalopods have internalised their mollusk shell, such as cuttlefish and squid. Many cephalopods are able to squirt ink. Class Cephalopoda has a long evolutionary heritage, with extinct varieties such as the ammonites being common in marine fossil beds.
The Head-Feet: An article introducing the characteristics and biology of Class Cephalopoda.
Guide to Class Cephalopoda: An information resource on cephalopods, with photographs.
The Biology of Class Cephalopoda: An overview of the defining biological characteristics of the class.
What are Cephalopods?: An article examining the nature of Class Cephalopoda.
Octopus
The octopus is a member of the order Octopoda of the Class Cephalopoda. Octopuses possess four pairs of arms, with a hard beak and mouth at the center of them, and all species are venomous, though only one is fatal to humans. Many have no skeleton, allowing them to maneuver through even the tightest of gaps. Octopuses are renowned for their great intelligence amongst invertebrate species.
Octopus Image Gallery: A selection of photographs of octopuses in their natural environments.
What Does An Octopus Look Like?: An article examining the distinctive characteristics of the octopus.
Giant Octopus Fact Sheet: An in-depth look at the Giant Pacific Octopus, with photographs and illustrations.
Mollusks are animals with an enormous diversity of biological and visual characteristics. It is this ability to diversify over millions of years into new habitats and ecological niches that has led to mollusks to become one of the most successful families of animal life on Earth.
