A Parasitic Fungus

Pseudoboletus parasiticus on Scleroderma citrina, a Common Earth Ball.   Keston Common, 7 October 2012.

It's not only people and slugs who like eating fungi.  Other fungi do, too.  There are quite a few parasitic species, and while it's often not easy to tell what is going on, in this case it is obvious.  This bolete is growing out of the side of the Earth Ball it is parasitising.

The Earth Ball still looks healthy enough and will be full of spores, so it must have a good supply of nutrition if it is feeding that big mushroom as well as itself.