Arthropods

The body of an arthropod consists of a tough exoskeleton made of chitin. To let the animal move ( if it had to stay still in such armour, it would get stiff muscles), the animal's armour has a plate formation. There are joints between the feet. Some animals like the millipede and centipedes, they have two branches. The outer is a flattened gill, the other is to walk or grasp, chew or reproduce. Some have only single legs - like the scorpion and the spider, which have but one leg per appendage unlike the branched legs of their ancestors Generally, one branch was concealed.

These animals have a central cavity thrrough which blood flows and bathes the organs. They usually lack blood vessel like structure.

On the issue of what were the closest ancestors of this vast group of animals is debated. There was a time when the segmented worms were considered their relatives, but the chance of that being found anywhere as a universal rule is slim.