Spiders, ants, crabs, lobsters, and beetles. They may not be the prettiest bunch of animals, but they are the most plentiful. They are all arthropods. In the grand scheme of the classification of life they are in the Phylum Arthropoda. At the phylum level, it means the group generally shares similar body structures. (Humans are in the chordate phylum with all other vertabrates… everything with a back-bone)
Arthropods are recognized as having an exoskeleton, segmented bodies, and jointed appendages. (I suggest picturing a crab or lobster as you read this, because thinking this hard about spiders might cause the heebie-jeebies.) Having an exoskeleton simply means they have no bones inside their body. (Our bone structure would be an endoskeleton)
The tough outer shell is grown around the animal a bit like our fingernails. It provides the body structure, protects the soft bits inside, and is totally water proof allowing arthropods to survive in the driest places on earth. The primary ingredient of these shells is chitin. It’s a flexible and strong material. If you have surgery, chances are your stitches will be sewn with chitin thread. It holds tight, but will safely decompose over time.
Some exoskeletons are reinforced through a process of biomineralization, wherein calcium carbonate (the chief ingredient in rocks and eggshells) is bonded with the chitin to make it, literally, rock hard. Most arthropods are rather small, but having a stronger mineralized exoskeleton allows them to grow bigger. The largest arthropod is the Japanese Spider Crab that can reach 4 meters across… though it’s mostly legs.
Arthropods are also known for all the things that stick out of their bodies. Arms, legs, antenna, and other sensors. They’ve been called the swiss army knives of the animal kingdom. That exoskeleton offers much protection, but it also limits the sensitivity to the world around them, so all the antennne and sensors have worked their way outwards. Most anthropods have very primitive eyes, what they call a pit sensor, that can only tell the general direction light comes from. Rather they rely mostly on chemical and touch sensors.
There are a few vertebrates and other animals that have developed exoskeletons such as the armadillo and most famous of a shelled creatures, the turtle.
