Invertebrate zoology is the biological discipline that consists of the study of invertebrate animals, i.e. animals without a backbone (a structure which is found only in fish, amphibians, reptiles, birds and mammals.) Invertebrates are a vast and very diverse group of animals that includes sponges, echinoderms, tunicates, numerous different phyla of worms, molluscs, arthropods and many additional phyla. Single-celled organisms or protists are usually not included within the same group as invertebrates.