Sponges, Cnidarians, Mollusks, and Echinoderms are not Arthropods.
Arthropods belong to the animal (Animalia) kingdom, also known as Metazoa.
because it includes millions of species of insects
All arthopods do not have backbones which makes all arthopods invertebrates!
arthopods
An arthopod is not a 'thing' with a function within something... It is a an invertebrate animal having an exoskeleton (external skeleton), a segmented body, and jointed appendages.
Exoskeletons.
An arthropod is an invertebrate animal, such as an insect or spider. Most arthropods move by walking. However, some can move by swimming, flying, hopping and wiggling.
the normal ones
dragonflies belong to the animal kingdom they are part of the invertabrates they are part of the arthopods and are insects
Gill
they are all jointed
Arachnids such as spiders and scorpions have chelicerae.