Arthropoda is the largest phylum in the world, with over 1,000,000 known species. This group of creatures possess multiple common characteristics that justify them to be classified in the same group.
On the Linnaean scale of classification, phylum is the classification directly below the kingdom classification. Among arthropods, there are multiple different classes and sub-phyla. Some of these include: crustaceans, insects, and arachnids. Some common classes of arthropods are lobsters, crabs, beetles, winged insects, etc. Sub-phylum, chelicerata, contains the class arachnid, which includes spiders, ticks and scorpions.
There is tremendous diversity within this phylum, however, they all share a basic body plan. There are variations among different classes and species (and especially the sub-phylum arachnids), but they all follow one basic blueprint. Each further classification is specialized with different vices, but still share basic characteristics. What is this body plan? Arthropods have a full body exoskeleton, and jointed limbs, and three separate body cavities: a head, a thorax, and an abdomen. –With the exception of arachnids, that have a joined head and thorax, called a cephalothorax.
As many species as there are within this phylum, one can imagine the incredible diversity of arthropods. There is so much detail we already know, and there are so many details mankind has yet to discover.
All these species being classified into one group poses the question: should they all be classified in just one group? Yes, they should all be classified together. Why? Because they all share the same basic structure, and engage in similar behaviors. This body plan gives them similarity, and even though the basic structure varies tremendously, the species all have similar behaviors and metabolic processes. The body blue print of arthropods preserves the relationship between all the species within the phylum.