By what principle are animals divided into invertebrates and vertebrates?

Animals are divided into types of invertebrates and vertebrates according to the following principle: the former do not have a solid skeleton and spine. These include cephalopods (octopus, cuttlefish), insects (beetles, ants), worms. Insects also have a hard coating (chitin exoskeleton) that should not be confused with the spine and skeleton. Vertebrates are much smaller on the planet, but they are more developed and are located at the top of the hierarchical pyramid (mammals, amphibians, fish, amphibians, reptiles).