The Mollusca phylum is one of the most diverse phylums on the planet, with at least 100,000 living species. It includes organisms such as snails, slugs, clams, squid, scallops, and oysters. Molluscs have soft bodies and hard, outer shells to protect themselves. The Porifera phylum consists of several species of sponges, about 5,000 in total. Memebers of the Porifera phylum are among the simplest animals in the world. Sponges don't have a digestive tract, nerve cells, or muscle cells. They digest their food within cells. The Cnidaria phylum contains over 10,000 species of animals found in aquatic and marine environments. Animals within this phylum include coral, jellyfish, and sea anemones. The Platyhelminthes phylum consists of about 25,000 flatworms. Members of this phylum have soft bodies and are the largest phylum of animals without a body cavity. Flatworms can be found in marine, freshwater, and land environments. The Echinodermata phylum includes animals that live completely in the water, such as starfish and sea urchins. These animals live mostly on the ocean floor, although they are also commen near shore and on reefs. The Echinodermata phylum includes about 7,000 living species and about 13,000 extinct species. The Chordata phylum is made up of animals that have a notochord. A notochord is is a flexible-rod shaped body found in embryos of all chordates. These are over 100,000 animals within this phylum. The Annelida phylum is made up of invertebrate worms. These worms are segmented, meaning their bodies are divided into several sections. There are about 17,000 worms in the Annelida phylum. The Nematoda phylum consists of more than 500,000 roundworms. Nematodes are very similar to eachother, as their body structure is constant from species to species. The Arthropoda phylum is the most numerous phylum of all living organisms. An arthropod has a segmented body covered by an exoskeleton. An arthropod is also an invertebrate and has jointed appendages, which are external body parts that protrude from an organism's body.