World Builders™
World Builders™
Session Six  --  Water Animals
Session Six  --  Water Animals

Mollusca -- Clams, Snails,  and Octopi
 Mollusca -- Clams, Snails, and Octopi

Some animals grew shells to protect themselves from predators.

     The molluscs are a hardy and successful group of animals.  There are about 128,000 known species of molluscs.  For many of them, their life strategy is to protect themselves from predators by building protective shells of calcium carbonate, a mineral that is available in sea water. Some molluscs, however, have evolved away from exterior shells, reducing the size of the shell or eliminating it entirely.

     Some of the earliest fossils are made of "little shelly bits", fragments of the shells of what may have been early molluscs.  Shells are well preserved as fossils, but the soft tissues are not, so if all you have are bits of shells it is challenging to try to understand what animals made the shells.

     If you imagine a primitive animal as essentially a digestive tract, you wind up with a tiny worm with an inside and an outside.  Now there's a tempting morsel!!!  Toughening the outside of the worm with a little calcium carbonate could provide some protection against predators, and experimentation with the shell would allow the development of the molluscs that we see today.

     The molluscs are one of the only three phyla that includes members who colonized the land.  Molluscs can be found in fresh water and all depths of the oceans.  They are unsegmented animals and most have heads and eyes.  Mollluscs that are familiar to us include:

Snails and Slugs
Snails and slugs can be found on land and in both  fresh and ocean water.  The underside of these animals has developed into a strong "foot" which they use for locomotion.  Both snails and slugs have a strap- like structure called a radula which they use for scraping algae off rocks .   Most of these animals are herbivores, but a few are carnivores, eating other molluscs and other small animals.


     The animals will have shapes suited to their life strategies. Many simple animals are little more than tubes with openings at each end so that food can pass through and be digested. Such animals, though, are tender and fragile, a tasty morsel for a passing predator. Some of them will learn to hide in the mud or will grow various kinds of protective shells. Shells may limit movement, honautiluswever.

     On earth we see many sea animals that have simple shells with no moving parts. Some mollusks have only a single shell, some have shells like little tubes, some have coiled shells, snail shells.

This picture is of the Chambered Nautilus, a mollusk that can swim in the ocean. It is noted for its large, beautiful, spiral shell.  As the nautilus grows, it gets too large to live in the smaller part of the shell.  By filling that space with gas, the nautilus gains buoyancy so that it can float in the water and use its energy for swimming to wherever it wants to go.  This is an interesting adaptation!

Some mollusks have two shells that can be opened and closed. Clams and mussels are examples of bivalve mollusks



     Bivalves are molluscs with two shells, for example, clams and oysters.  The two shells are joined by a hinge, and can be closed and held closed by a powerful muscle.  In this way the shellfish can protect themselves from enemies

mussels and oysters



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Photos from archives at Biology Department, University of Bowling Green, Ohio
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© 1996,1997, 1998, 1999, 2000, 2002, 2003.   Elizabeth Anne Viau. All rights reserved. This material may be used by individuals for instructional purposes but not sold. Please inform the author if you use it at .