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.
Balantidium coli has 2 contractile vacuoles. Although contractile vacuoles are common to ciliates, they are rare in parasitic protozoa, which suggests that Balantidium coli has a unique osmoregulatory capacity. Balantidium coli has 2 developmental stages: a trophozoite stage and a cyst stage.