What are Mollusks?
Mollusks are animals like mussels, clams, snails and slugs. Cephalopods like octopus and squid are also mollusks. The name Mollusk means "soft", because mollusks don't have any skeletons. The first mollusks probably evolved from early segmented worms around 500 million years ago. At that time, all animals except arthropods still lived in the ocean, and all these early mollusks also lived in the ocean.
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.
But mollusks also have some things that segmented worms don't have. Many mollusks have shells made of calcium to protect them.
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. The earliest evidence of land snails comes from about 325 million years ago, 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.
Bibliography and further reading about slugs and snails:
Karen Eva Carr, PhD.
Assoc. Professor Emerita, History
Portland State University
Professor Carr holds a B.A. with high honors from Cornell University in classics and archaeology, and her M.A. and PhD. from the University of Michigan in Classical Art and Archaeology. She has excavated in Scotland, Cyprus, Greece, Israel, and Tunisia, and she has been teaching history to university students for a very long time.
More about Professor Carr's work on the Portland State University website
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