The Phylum Arthropoda is divided into seven different classes. Two of them include Merostomata and Crustacea. These two classes include organisms such as horseshoe crabs, lobsters, shrimps, and barnacles. This is the most successful group of animals since almost 75% of all identified animal species belong to this Phylum! One big contributor to their success is the evolution of an exoskeleton that is composed of proteins and chitin.