Scientists often categorize animal phyla into about thirty-five divisions—whether they are found in the oceans, on land, or in between. The nine major phyla include Porifera (sponges), Cnidaria (hydra and jellyfish), Mollusca (mollusks), Echinodermata (starfish and sea urchins), Nematoda (roundworms), Annelida (segmented worms), Platyhelminthes (flatworms), Arthropoda (insects, crustaceans, and arachnids), and Chordata (fish, amphibians, reptiles, birds, and mammals).