Cephalodia (s. cephalodium) are structures on lichens containing two photobionts (both an algae and a cyanobacteria).  They house the cyanobacteria and usually appear as black lumps on the surface of a lichen.

A cross-sectional view of a cephalodium from Peltigera leucophlebia, showing isolation of the cyanobacterial partner in the thallus (~100X). (A. Fischer--3/2011)

The cephalodia on Peltigera leucophlebia. (A. Fischer--3/2011)