This is made possible through something called a lenticular lens – a corrugated sheet in which each ridge is made up of a convex – or outward-curving – lens.
The core is even hotter than the mantle and, as the descending sea floor reaches its outer surface, it's superheated and rises back up in vast plumes towards the surface.
From your stomach on down, though, the inner GI tract is lined with simple columnar epithelial cells, which secrete all sorts of stuff, and which absorb and process various nutrients.