Echinoderms are unique among the 35 animal groups, as they are only found in salt water, usually clinging to the sea bottom. The name comes from the ancient Greek "echinos" for spiny and "derma" for skin. You are probably familiar with some of the animals in this group, which includes sea stars, sand dollars, sea cucumbers and urchins.
Echinoderms are radially symmetrical, meaning no matter where they are divided in half, the two pieces will be the same. We are bilaterally symmetrical, meaning that only by drawing a line down the center could we be divided into matching halves.
Dividing an echinoderm in half is no big deal because many of them have the ability to regenerate tissue, organs or even a new animal from just a limb. Some fishermen who found sea stars eating things caught in their traps cut them in half, thinking that would kill them. Instead, many would grow into two whole new stars. It helps to know the biology of an animal before trying to work with it.
Sea stars -- biologists don't call them starfish because they are not fish -- have a tiny mouth that can eat whole clams or mussels. The sea stars do this by pushing their stomach out through their mouth, digesting the captive into a liquid, then pulling it all back in. Imagine doing this to your next meal. You'd have to leave yourself a lot of time because this can take hours.
Urchins use their spines for movement or defense. Our largest urchin has long sharp spines, which can easily penetrate skin where their barbs get caught and the spine breaks off. The worst part is the mild toxin that makes this puncture wound hurt. But if the wounded animal has a brain, it will learn not to touch an urchin in the future, which is just what the urchin wants to happen.