The oceans harbor a wide variety of living organisms, each exhibiting adaptations for survival in their particular marine niches. The relationship between structure and function in the living organisms studied this semester is the cells that they have in their body. The way that these creatures act, and hunt for food. Their body systems have critical functions, such as locomotion, reproduction, digestion, and circulation. All living things on Earth are composed of the same carbon based, molecular building blocks. All organisms are composed of cells. Organisms can be single celled or multicellular. Now comparing and contrasting members of the major marine animal phyla in terms of structure, adaptation, lifestyle, and evolutionary history is a list of animal phyla is a list of the major groups of animals usually classified as a phylum. A list of this type may be arranged alphabetically, but equally it might be arranged according to evolutionary relationships. No list will be completely satisfactory. Authorities differ in what they consider a phylum, and in the actual name of the phylum. Despite this, there is agreement on most phyla. Most modern surveys include groups above the phylum superphyletic groupings based on evidence of common descent. Animals in the Phylum Chordata include the vertebrates and some of the more primitive nonvertebrates like the protochordates, lancelets, acorn worms, tunicates, and the pterobranchs. The first vertebrates appearing in the fossil record during the Cambrian age were animals that resembled fishes and had respiratory gills formed by pharyngeal gill slits located in a set of pouches. The first purpose of the skeleton and scales were to protect the animal, to add support to the notochord, and to keep the brain protected. Jawless fish lived from the Late Cambrian until the end of the Devonian period. These fish were covered in bony armor, an adaptation that helped protect them from other animals. Parasitic lampreys and deep-sea hagfish are descended from the weak swimming, bottom dwelling jawless fish. Later in the Middle Silurian, a fish with jaws and teeth, known as the vertebrate, evolved.