Phylum: Arthropods

Michael and Leon are hiking, and the forest around them is buzzing with life. There are spiders spinning webs between tree branches, ants making their way back to the massive anthills, crayfish in the river flowing nearby, and when Leon moves a log off the path, it reveals millipedes and centipedes underneath. It might be hard to believe but all these animals have a lot in common! They belong to five categories or classes - arachnids, insects, crustaceans, millipedes, and centipedes. Together they make up a group of organisms that have a similar body and development - we call such a group a PHYLUM. - But, their bodies don’t look similar at all!

Let’s look closer at some of these animals - a spider, an ant, a crayfish, a millipede and a centipede; and you’ll find that they’re more similar than they seem at first! First imagine a line dividing each of these animals into a left and right side - note that both sides are symmetric! We call this BILATERAL SYMMETRY. Another thing that stands out, is that the bodies of these animals seem to be divided into smaller parts, or SEGMENTS. For some of these animals, the main body segments are head, thorax and abdomen.

But some animals, like the spider, only have two - the abdomen and a combined head and thorax - cephalothorax. Now, let’s look at the external body parts - legs, and antennae also called APPENDAGES. They are also made of segments, connected together by JOINTS. These ‘jointed appendages’ are characteristic of the phylum that all these animals belong to - ARTHROPODS. - Oh I see! They do seem more similar now that we look at them this way! - And what’s that?

It looks like a crayfish ghost, all white and empty! It’s not a ghost Leon! It’s the hard outer part, like a shell that covers the bodies of all arthropods - the EXOSKELETON. It protects the internal tissues from damage, prevents dehydration and provides a surface for muscles to attach to. But because the exoskeleton is so hard, it’s like a jumper - it doesn’t grow together with the organism “wearing” it.

When the crayfish grows, it sheds the old exoskeleton. A bigger exoskeleton has grown in its place. So what Leon pointed out as a ghost was actually just an old crayfish exoskeleton! This process of shedding the exoskeleton is called MOULTING, and all arthropods do it! Now we know what makes all these organisms arthropods.

Arthropods are the largest phylum in the animal kingdom - they represent 80% of all animal species living on Earth today! But arthropods are some of the most endangered animals and a lot of species face extinction because of human activity. They might not be as charming as some of the bigger animals, like polar bears or bunnies, but arthropods deserve just as much attention and protection. They are extremely important parts of ecosystems and are also essential for the survival of humans.