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How would you design the most simple land animal imaginable, the main requirements being that it could move about and eat, protect itself reasonably well, and reproduce?
First of all you'd give it some kind of foot, but only one, to keep it simple. You'd dump all the basic organs into a fleshy, bag-like body, not bothering with complicated matters such as a skeleton and a complex network of interconnected blood-veins. Finally, such a creature wouldn't have much intelligence or fancy self-defense mechanisms so you'd just place a simple shell over its soft parts amd give it just enough brainpower so that when the creature felt threatened it'd automatically withdraw into its shell.
Once you'd accomplished this, basically you'd have a mollusk -- a member of the primitive phylum Mollusca. At the right, that's a Giant African Land Snail, a good example of a mollusk. The best-known mollusks are snails and slugs, clams and oysters, octopuses and squids. Aquatic mollusk fossils are known from rocks deposited long before any land animal existed.
Although at first glance there's a world of difference between a slug and a clam, or a clam and an octopus, these creatures are fundamentally alike in being composed of these three main body parts:
You know that in humans the heart pumps oxygen-rich blood from the lungs, through arteries, to all parts of the body where tiny capillaries carry the oxygen to where it's needed. Then veins return the oxygen-depleted blood back to the heart and lungs, where it is enriched with oxygen, and recycled again. In humans, blood is always in the process of being transported someplace as if it were in a system of pipes.
In mollusks, in contrast, a very simple heart pumps blood into open body cavities, or sinuses. It's a system a little like pouring water into buckets filled with stones, with the buckets being the sinuses, and the stones being organs. Blood simply washes over the organs. As blood enters the sinuses, other blood in the sinuses fairly haphazardly drains into vessels, which typically transport the blood to gills, where the blood is recharged with oxygen. Then the blood returns to the heart and is pumped to more sinuses.
Though the mollusk circulatory system seems almost comically simple when compared to ours, it seems to work well for slow-moving critters like most mollusks. In fact, it works well enough for mollusks to constitute the second-most-diverse phylum of animals on Earth -- (the most diverse being the insects' arthropod phylum).
About 150,000 living mollusk species are known, and thousands of now-extinct fossil forms have been described. Mollusks range in size from 60-foot Giant Squids, down to almost microscopic species.
One limitation of the mollusk manner of being is that it works better in water than on dry land. The vast majority of mollusks are aquatic. The best known land mollusks are snails and slugs, and of course these even appear in most backyards from time to time, especially in moist areas.
Here are the main subdivisions of the Mollusk Phyllum. (To understand better about concepts like phyllum and class, check out our Classification Page.)
To see a much more complete, and confusing, breakdown, visit the NCBI taxonomy database page for mollusks
On the Web check out the mollusk page of the The Tree of Life, which provides an on overview of mollusks, from a taxonomical perspective. Endangered mollusks in the U.S. is worth visiting, and the U.S. Geological Survey provides a fascinating page on fossil mollusks.
You may be interested in reviewing books about mollusks available at Amazon.com by clicking here.