Molluscs are one of the most successful of all animal groups. They first appeared during the Cambrian Period. Most molluscs had a hard rock-like shell covering their soft bodies.
Snail image courtesy of Mad Max
Some, such as the snail, crawled along. Others, like oysters, had a hinged shell which opened like a box. Yet others lost their hard shell, like the octopus. To get enough energy to move, molluscs and most other animals have an organ called a gill. This absorbs from the water and gives out carbon dioxide.
Arthropods (“arth-ro-pods”) were another major animal group which appeared during the Cambrian. They were the commonest animals then and have been ever since. They played a key role in the history of life.
Arthropods have jointed legs and bodies covered by hard skin (exoskeleton) made of protein and compound eyes.
Origin of Arthropods
Eurypterid or sea scorpion
Arthropods first appeared around 540 mya during the period. Early arthropods included crabs, spiders and extinct animals like the sea scorpion shown above. Today there are more types of arthropod than all the other types of animals put together.
To grow, arthropods had to shed their hard skin, pump themselves up with air or water, and grow another skin. This way of growing was wasteful, and probably limited the size that they could grow to. So although they are the commonest animals on Earth, they have never been a dominant type of animal. However they have played a major role in evolution. For example probably could not have evolved without them.
, arthropods and all the other animals we have seen so far had no . They are called invertebrates.
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