Vetigastropods typically feed on such organisms as bryozoans, tunicates, and sponges. It feeds on sponges, algae, bryozoans, zoantharians, gorgonians and tunicates. These include tunicates, mollusks and sessile Cnidarians. Their diet consists primarily of crustaceans, molluscs, tunicates, and other fish, particularly stingrays. The little tunny is an opportunistic predator, feeding on crustaceans, clupeid fishes, squids, and tunicates. Scallops and cardiids, siphonophores, tunicates (such as salps), and some jellyfish also employ jet propulsion. Leatherbacks also feed on other soft-bodied organisms, such as tunicates and cephalopods. Ernst Haeckel described a taxon comprising tunicates, cephalochordates, and vertebrates in 1866. A hollow nerve cord is found in all chordates, including tunicates (in the larval stage). The clade consisting of tunicates and vertebrates is called Olfactores. The Thaliacea, the other main class of tunicates, is characterised by free-swimming, pelagic individuals. Some tunicates are used as model organisms. The appendicularian body shape resembles that of the tadpole-like larvae of most tunicates. Sea tulips are tunicates with colourful bodies supported on slender stalks. This genus is found in the renal sac of molgulid ascidian tunicates.