Arthropods are among the most disgusting animals around. Insects from ants to roaches. Millipedes, centipedes, scorpions, crabs, lobsters, crayfish are all arthropods.
Evolutionary, arthropods are notable because they were the first animals to inhabit land. They were preadapted to land because they had hard shells for support and to slow dehydration, an excretory system that conserved water and a good respiratory system. They hit land at about 400 MYA and exploded from there. Even today, Phylum Arthropoda has more species than any other phylum.
Arthropods are animals, so they go in Kingdom Animalia. They have organs so they go in Subkingdom Eumetazoa. They have bilateral symmetry, so they go in Branch Bilateria. They have true body cavities so they go in Grade Coelamata. They are protostomes and have spiral, determinate cleavage so they are placed in Subgrade Schizocoela.
The classification scheme breaks down like this:
All arthropods have:
What you should know is this: Arthropods evolved 400 MYA and were the first animals to inhabit land. They were able to do this because of their excretory system, their exoskeleton and their respiratory system. Know the classifications and why they go in the spots they do.
Know that arthropods are protostomes, so they have spiral, determinate cleavage and that the blastopore becomes their mouth.
Know that the green gland of a crayfish (organ used for excretion of urine) is ANALOGOUS to the Malpighian tubules of the grasshopper. Know that crayfish and grasshoppers display sexual dimorphism - males look different than females. A female grasshopper has an ovipositor and a male crayfish has enlarged swimmerets. Male grayfish also have a genital pore but it is not obvious.