Extending outwards from the core are the mantle and the crust. Earth's mantle extends to a depth of 2,890 km beneath the surface, making it the thickest layer of Earth. This layer is composed of silicate rocks that are rich in iron and magnesium relative to the overlying crust.
The Parasite: Balantidium coli. Balantidium coli is the only ciliate known to parasitize humans. Ciliates represent a phylum of protozoa characterized, in at least one stage of development, by simple or compound ciliary organelles on the surface of their membranes that are used for locomotion.
The life cycle of Balantidium coli begins, when a human eats food or water that has been contaminated with infective cysts. If the cysts survive through the stomach, trophozoites are formed in the small intestine. Trophozoites live in the cecum and the colon of the large intestine.