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Phylum Arthropoda is varied and widely spread.

However, all its members share certain characteristics. In this lesson you’ll learn about one of these: the exoskeleton.

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Phylum Arthropod

Imagine if, instead of your skeleton being inside your body, it was on the outside instead! That’s exactly what life is like for members of phylum Arthropoda. Arthropods make up the largest and most varied phylum, including insects, arachnids, and even crustaceans (the group including crabs). They exist in basically all land- and ocean-based ecosystems, so they can be found across the globe. Despite the wide variety of this phylum, all of its members share certain characteristics.

One of these is the exoskeleton.

Exoskeleton Characteristics

The exoskeleton of an arthropod is their tough, hard outer shell. It is made up mostly of chitin, which is a very strong material related to cellulose. Unlike turtle or snail shells, it does not grow as the arthropod gets bigger. Instead, the animal has to molt, or shed its old skeleton after growing a new, larger one underneath it. Can you imagine if every time you had a growth spurt you had to step out of your old skin and grow a completely new one?

A cicada breaks out of its old exoskeleton as it grows
Molting cicada

Though chitin is a tough substance to begin with, many arthropods add calcium carbonate to their exoskeleton to make it even stronger.

This is especially true for crustaceans, such as crabs and lobsters. Calcium carbonate is a very hard substance found in rocks like limestone and marble. This addition makes the animal far less vulnerable to a predator’s bite.

Pros and Cons

As with any system, having an exoskeleton comes with advantages and disadvantages. One positive is that it provides an excellent protective covering for soft internal organs and muscles. Creatures without exoskeletons, like humans, are more vulnerable to muscle and organ injury.In addition, the exoskeleton is made up of layers.

The harder outer layer and the more flexible inner layer work together to provide even more protection against predators. The waxy innermost layer protects against dehydration, and many arthropods have another layer that prevents the waxy layer from being torn or damaged. All these work together to make an efficient system of protection.One major disadvantage is the need to molt. Arthropods are highly vulnerable during this time, since the new exoskeleton starts out very soft and needs some time to harden. Until it hardens completely, the animal has almost no protection.

Historical Significance

Arthropods (and their exoskeletons) have been around since the Cambrian period, which started about 543 million years ago. This was the first era where major evolution of different life forms is thought to have begun. Arthropod-type life forms are considered among the oldest complex life forms on earth.

The makeup of exoskeletons means they fossilize more easily than other types of animals. Therefore, a lot of what is understood about early eras like the Cambrian period is related to this type of animal. That makes them extremely scientifically significant. It also means that scientists can study how similar early arthropods were to current ones, which helps them learn more about evolutionary processes.

Trilobites from the Cambrian period are an ancient form of arthropod
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