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Arthropods have a number of features that set them apart from other animals. One of these features is their open circulatory system, with both a haemocoel and hemolymph, which we’ll discuss in more detail in this lesson.

What Are Arthropods?

Picture a lobster for a second. You should be able to picture that hard, red shell on the outside of its body. It is that exoskeleton that classifies it as belonging to the phylum Arthropoda.Arthropods (animals in the phylum Arthropoda) have segmented bodies as well as an exoskeleton.

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They include animals such as insects, arachnids (spiders, scorpions, etc), and crustaceans. So, if arthropods’ skeletons are on the outside, is what’s on the inside different too? Sure is, but there are some things you’ll find familiar. Let’s take a look at their circulatory system.

Circulatory System

You can imagine that if you had a skeleton on the outside, your circulatory system could be much different. Arthropods have what is referred to as an open circulatory system, where blood fills the body cavity of the animal. Humans have a closed circulatory system, where blood is moved around the body via vessels and arteries.Inside of many animals exists an area of empty space called the coelom where you can find its organs. A coelom helps to give animals some rigidity and acts like a skeleton, giving the body a sort of structure. In arthropods, this coelom is much reduced, and divided into smaller areas around excretory and reproductive organs.

Since they have skeletons, the leftover space is instead filled with blood which covers the other organs, keeping them bathed in blood. This cavity is referred to as a haemocoel, or blood cavity.

Blood fills the empty space in this image of a spider
Spider cutaway

Blood Flow In Arthropods

The type of blood that is in arthropods is also different from what a human would consider ‘normal’ blood. It doesn’t have the oxygen content that humans do.

However, it does help to distribute the oxygen and remove carbon dioxide throughout the body similar in scope to a human’s blood. Arthropod blood is copper based, so it does not appear red like a human’s. We refer to this blood-type fluid inside of the system as the hemolymph.

A basic example of the open circulatory system
Open circulatory system

The blood is pumped along the haemocoel via the animal’s heart.

This muscle ripples instead of contracting, pumping the blood forward in the animal. It then is cycled around again allowing them to move oxygen around the body to different organ systems.Some arthropods have lungs, some have gills, and small ones will diffuse oxygen through their skin. Recall that diffusion is the movement of a molecule or substance across a membrane from a higher concentration to a lower concentration. This allows these arthropods to breathe.

How Is This Important?

An arthropod’s circulatory system is fairly streamlined and simple. Studying it helps us to understand our own systems and their evolutionary history.

Scientists can study the open circulatory system of arthropods and see how it varies and developed into the closed system that we as humans have developed.

Lesson Summary

Arthropods are a phylum that consist of animals with segmented bodies and exoskeletons. Because they have a skeleton, they have no need for the typical coelom (fluid filled body cavity found in many other types of animals). Instead they have a haemocoel, a cavity that is divided into smaller sections and filled with blood.

This exists around the main organs of the animal and keep those organs bathed in blood.Arthropod blood, called hemolymph is copper based and helps distribute oxygen and remove carbon dioxide. It’s pumped via the heart, which ripples instead of pumps, through the haemocoel of the animal. Depending on the particular animal, lungs, gills, or diffusion act to draw in oxygen and remove carbon dioxide.

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