While arthropods and roundworms exceed the phylum Mollusca in terms of species, molluscs hold their own when it comes to anatomical diversity. There may be well over a million species of arthropod, but crabs, spiders and bees are all obviously related, sharing the same multi-limbed body plan organised around a jointed exoskeleton. Molluscs are very different. Clams, snails and squid are all molluscs, but their anatomy, ecology and behaviour couldn’t be more different.
What molluscs have in common
Although incredibly diverse, molluscs do have features in common. These include:
- A fleshy foot used for locomotion.
- A visceral mass containing the internal organs.
- A mantle that secretes the shell.
- A toothy tongue, known as a radula, for scraping food into smaller pieces.
- A shell made from calcium carbonate.
Not all molluscs have all of these features, but they each have at least some of them. So, while an octopus doesn’t have a shell, it does have a mantle and a radula, as well as a foot divided up into the eight arms that give it its name.
From the perspective of the palaeontologist, the key thing about molluscs is that most have (or had) shells. These fossilise more readily than soft tissues or even bones, and that means that molluscs have a remarkably rich fossil record.
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