Larva refers to the stage in the life cycle of certain organisms between the egg and adult. These juveniles do not resemble the adult, but undergo a morphological transformation into the adult stage. This type of indirect development is termed metamorphosis. In several groups of sessile (attached) marine organisms, the larval stage functions in dispersal. Examples of these include the planula larva of sponges and cnidarians, the pluteus larvae of echinoderms, the nauplius and zooea larvae of arthropods, and the trochophore and veliger larvae of certain molluscs. These structures are typically propelled by cilia and may travel with water currents over long distances. The veliger larva of zebra mussels (Dreissena polymorpha) are suspected to have been transported from northern Europe to Lakes Erie and Huron in 1986 by tanker ships emptying their ballast water. These small molluscs have now invaded much of the Great Lakes watershed and cause hundreds of millions of dollars in damage each year by clogging water intake pipes. Some parasitic animals have intermediate larval stages that mature in vectors and intermediate hosts before entering humans and other vertebrate final hosts. Examples of these include the miracidia and cercaria larvae of trematode flukes. Many insects also have a larval stage, which are known in various groups by different names such as caterpillars (moths and butterflies), grubs (beetles), maggots (flies), and nymphs or naiads (aquatic insects). A tadpole stage of the frog is also considered to be a larval stage.