Invertebrate zoology is the subdiscipline of zoology that consists of the study of invertebrates, 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. Subdivisions Invertebrates represent 97% of all named animal species, and because of that fact, this subdivision of zoology has many further subdivisions, including but not limited to: Arthropodology - the study of arthropods, which includes Arachnology - the study of spiders and other arachnids Entomology - the study of insects Carcinology - the study of crustaceans Myriapodology - the study of centipedes, millipedes, and other myriapods Cnidariology - the study of Cnidaria Helminthology - the study of parasitic worms. Malacology - the study of mollusks, which includes Conchology - the study of Mollusk shells. Limacology - the study of slugs. Teuthology - the study of cephalopods. Invertebrate paleontology - the study of fossil invertebrates These divisions are sometimes further divided into more specific specialties. For example, within arachnology, acarology is the study of mites and ticks; within entomology, lepidoptery is the study of butterflies and moths, myrmecology is the study of ants and so on. Marine invertebrates are all those invertebrates that exist in marine habitats. History Early Modern Era In the early modern period starting in the late 16th century, invertebrate zoology saw growth in the number of publications made and improvement in the experimental practices associated with the field. One of the major works to be published in the area of zoology was Conrad Gessner's Historia animalium, which was published in numerous editions from 1551 to 1587. Though it was a work more generally addressing zoology in the large sense, it did contain information on insect life. Much of the information came from older works; Gessner restated the work of Pliny the Elder and Aristotle while mixing old knowledge of the natural history of insects with his own observations. With the invention of the Microscope in 1599 came a new way of observing the small creatures that fall under the umbrella of invertebrate. Robert Hooke, who worked out of the Royal Society in England, conducted observation of insects—including some of their larval forms—and other invertebrates, such as