Mollusk

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 oxygen from the water and gives out carbon dioxide.

Arthropods

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 Cambrian 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.

Arthropod Growth

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 flowering plants probably could not have evolved without them.

Molluscs, arthropods and all the other animals we have seen so far had no backbone. They are called invertebrates.