From the jet-propelled (but very slow moving) chambered nautilus to the poison-producing cone shell, diversity within the molluscs is remarkable. Most molluscs have an internal or an external shell, a muscular foot, and an unsegmented, soft body. A mantle, which is a fold of the body wall, lines the external shell and secretes the calcium carbonate and protein of which the shell is made. A tubular extension of the mantle called the siphon, when present, directs cilia-generated water currents into the mantle cavity. The current carries food as well as dissolved gases to the gills of the mollusc. Some molluscs gather food by boring or scraping with a hard, chitinous ribbon called the radula, a unique molluscan structure. Other molluscan species are predators; gastropod Charonia consumes sea star Oreaster. Still other molluscs live as parasites or commensals with sea squirts (ascidians, Phylum Urochordata ), annelids (Phylum Annelida), echinoderms (Phylum Echinodermata), or crustaceans (Phylum Crustacea). Molluscs fill an extraordinary array of ecological niches: mud and sand flats, forests, soil, fresh water, deserts, driftwood, and the abyss of the sea.