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 exists in either of two developmental stages: Trophozoites and Cysts. In the trophozoite form, they can be oblong or spherical, and are typically 30 to 150 Âµm in length and 25 to 120 Âµm in width. It is its size at this stage that allows Balantidium coli to be characterized as the largest protozoan parasite of humans.