Phylum Arthropoda are organisms which have jointed appendages and include: Crustaceans, Insects, Spiders to name a few. They are invertebrates.They are protostomes. This refers to an animal whose mouth is formed from the blastopore during embryonic development. Most invertebrates are protostomes. Deuterostomes refer to an animal whose anus is formed from the blastopore and later, the mouth forms...Echinoderms which are sea stars, sea cucumbers and all vertebrates--animals with backbones, develop in this way and are deuterostomes.
Most all invertebrates, including arthropods, are protostomes. Echinoderms and invertebrate chordates are the exception and are deuterostomes.
On the basis of embryological development the Metazoa are divided into two main evolutionary lines. One line contains the flatworms, annelids, molluscs, arthropods, and several smaller phyla, they constitute the division known as Protostomia (protostomes). From the other line have evolved the echinoderms, chordates and several smaller phyla, they are known as Deuterostomia ( deuterostomes). Each line displays a plan of development distinct from the other, though all members of each group do not have an identical pattern of development and they are many modifications in every phylum mainly due to the distribution and the amount of yolk present in the egg.