Why aren't Arthropods consider to be tetrapods?

Arthropods are not considered tetrapods because they belong to a different phylum, Arthropoda, which includes invertebrates like insects, arachnids, and crustaceans. Tetrapods, on the other hand, are classified within the phylum Chordata and include vertebrates such as amphibians, reptiles, birds, and mammals that have four limbs. The key differences lie in their body structures, developmental patterns, and evolutionary lineages, with arthropods having exoskeletons and jointed appendages, while tetrapods possess an internal skeleton and limbs adapted for terrestrial life.