The phylum Arthropoda, which directly translates to joint-footed animals, are segmented invertebrates. Which means that their bodies are divided into very distinguishable segments, and invertebrates tells us that these creatures have no backbones.
They are segmented invertebrates with a chitinous exoskeleton, so instead of having bones, they have an exoskeleton. Somewhat like a shell, or a hard outer covering on their body that is made primarily of chitin, and they have jointed walking legs and jointed other appendages, so that’s where the joint-footed came from that Arthropoda translates to.
Let’s look at a little bit more about arthropods. Arthropods have an open circulatory system, and what that means is that the blood comes directly into contact with the internal organs in the body cavity.
Where humans (and a lot of other creatures) have a closed circulatory system, where there are vessels, veins, arteries in your body that carry the blood to the heart for oxygen and then back away to oxygenate the body, the arthropods don’t have that.
Everything kind of has a round about place where it’s going to be in the body, but the organs all just sit in this open body cavity, and the blood kind of washes over them, and that’s how they get the oxygen they need instead of having veins or arteries that connect directly to each organ (so everything’s open there’s just the open body cavity with blood washing over the organs to give them the oxygen they need).
Due to their rigid exoskeletons, arthropods must molt periodically so that they can grow larger. They’ve got this hard exoskeleton and it’s not going to let them get any bigger than that exoskeleton is, so every once in a while, their exoskeleton will kind of crack open and fall off of them and then they’ll grow a new exoskeleton.
Whenever their exoskeleton cracks and falls off of them, that’s them molting their exoskeleton, and they grow a new one that will better accommodate the body size that they have, and allow them to grow a little bit larger before they have to molt again.
Some examples of arthropods are arachnids, which include your spiders, scorpions, and the horseshoe crab, which actually does fit in with arachnids better than crustaceans. Your myriapods are centipedes, and millipedes (the ones with lots and lots of jointed legs).
Insects, I’m sure you could think of lots of examples for those, bees, mosquitoes, ants, beetles (any of those). Then crustaceans would be your crabs, (not the horseshoe crab, but your other crabs that you would find out in the ocean) lobsters, and shrimp.
There are more examples for most of these, but just to give you an idea of what each type of arthropod is. That’s what these are, arachnids are your spiders, and scorpion type creatures, myriapods are the ones with lots of legs, your centipedes and millipedes.
Insects are the ones you might be able to best recognize and list out, bees, mosquitos, ants, beetles, and then your crustaceans, mainly ones that are going to live in the ocean, crabs, lobsters, shrimp.
Then the trilobites are actually extinct, but they are a member of Arthropoda and they left behind lots of fossils, so a lot of fossils that you see (like an example fossil of an animal) you’ll see a trilobite, which kind of looks like a beetley thing with a bunch of legs.
It was a creature with a segmented body, no backbone, with an exoskeleton, and jointed walking legs and other appendages. Those are the trilobites, they aren’t around anymore they are extinct, but there are a lot of fossils that show us trilobites.
One of the reasons that there are so many good fossils for arthropods is because they shed that exoskeleton, so instead of having just one body as a fossil, an arthropod would have one body plus all those exoskeletons that they molted and left somewhere.
Scientists can go and look at these fossils and see a lot more about arthropods from the past, then creatures that only had one body and never shed an exoskeleton to give extra chances for you to find a fossil.
Whenever you’re thinking of arthropods, or creatures from the phylum Arthropoda, remember they are joint-footed animals, (and Arthropoda translates to joint-footed) that have segmented bodies, no backbone, and a chitinous exoskeleton.