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a black slug on the grass

Evolution of mollusks: Slugs are a kind of mollusk.

When did the first mollusks appear?

Mollusks are animals like mussels, clams, snails and slugs. Their name means “soft”, because mollusks don’t have any skeletons. The first mollusks probably evolved from early segmented worms around 500 million years ago.  That was in the Cambrian period. At that time, all animals except arthropods still lived in the ocean, and all these early mollusks also lived in the ocean.

Segmented worms
The Cambrian period
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Like segmented worms, mollusks were generally long and thin, and they had a mouth and an anus with a long gut in between to digest their food.

Evolution of digestion

A snail with a red and black shell

Evolution of mollusks: Snails are a kind of mollusk.

Evolution of mollusks from worms

But mollusks also have some things that segmented worms don’t have. Many mollusks have shells made of calcium to protect them.

What is calcium?
A project with calcium

Cephalopods: octopus and squid

octopus in the ocean

This is an octopus – it’s a kind of cephalopod, and cephalopods are a kind of mollusk.

Cephalopods like octopus and squid are also mollusks, and also live in the ocean. “Cephalopod” means “headfoot” in Greek, because cephalopods like octopuses don’t really have bodies, just heads and feet. Like the earlier worms and arthropods, all cephalopods have bilateral symmetry.

What is bilateral symmetry?
A project with bilateral symmetry

Mollusks on land

Most mollusks still live in the ocean, but a few kinds of snails and slugs have evolved to live on land. Snails have probably evolved to live on land not just once, but on several different occasions, independently.

Why frogs and birds eat snails

The earliest evidence of land snails comes from about 325 million years ago. That was the Carboniferous period. around the same time that frogs came out of the water, and so frogs love to eat snails. But there weren’t so many land snails and slugs until about 145 million years ago, in the time of the last dinosaurs and the first birds – and birds also love to eat snails.

Learn by doing: cephalopods and mollusks
More about the Cambrian period
More about the Carboniferous period
And more about fish

Bibliography and further reading about slugs and snails and other mollusks:

More about the Cambrian period
More about the Carboniferous periodAnd more about fish
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