Members of the phylum Annelida are invertebrates with segmented, limbless bodies. It includes more than 16,000 species of worms, such as earthworms, leeches and clam worms. With round bodies, most members of the phylum Nematoda are human parasites, including Ascaris lumbricoides; filarias, which causes a disease called elephantiasis; and hookworms, responsible for ancylostomiasis. Platyhelminthes, from the Greek for "flat worm," have unsegmented, invertebrate flat bodies. Unlike other worms, Platyhelminthes do not have digestive, circulatory and respiratory organs; food and oxygen pass by diffusion into their bodies.
With more than one million described species, the phylum Arthropoda represents the majority of living animal species. The main characteristics of the group are the presence of a chitin exoskeleton and articulated limbs. Other common characteristics include an open circulatory system, where blood mixes with other fluids, and the molting of the exoskeleton throughout their life cycle. Insects, spiders, scorpions, centipedes and crustaceans (lobsters, shrimps and crabs) are members of the phylum Arthropoda.
The phylum Echinodermata (from the Greek for "spiny skin") contains about 7,000 species of marine animals, including starfish, sea cucumbers, brittle stars, sand dollars and sea urchins. The phylum's main characteristic is a body with radial symmetry and calcified spines. Jellyfish and other members of the phylum Cnidaria, which are also aquatic and radial in body symmetry, differ from Echinodermata because they contain specialized cells with irritating toxins. These cnydocites' or nettles' cells are used for defense and to capture prey.
Chordata include vertebrate animals, from fish to mammals. They have bilateral body symmetry, a dorsal neural tube that communicates with a well-developed nervous system, as well as other specialized circulatory, digestive and respiratory organs. With more than 98,000 species living in most habitats, members of the phylum Chordata are the most highly evolved animals.
Snails, slugs, mussels, squid and octopi are members of the phylum Mollusca, from the Latin for "soft body." With more than 100,000 species, it is the largest marine animal group, although some species live in freshwater or land habitats. Mollusca's main characteristics include an unsegmented soft body, which is sometimes shell-protected.