Below this is the algal layer where the photobiont lives. Below this is the medulla an area of loose hyphae in which nutrients are stored. Sometimes a lower cortex exists, in others the medulla rests on the surface. In crustose and squamulose lichens there is no lower cortex. In foliose lichens there is a lower cortex and in fruticose lichens the lower cortex is replaced by a central cone.
Filamentous lichens are totally different.
They consist of chains of algal cells wrapped around with fungal hyphae. Because nature was not designed to fit into our classification schemes inevitably some things to do not fit. In lichens it is the genus Cladonia. Cladonia are a successful group of lichens which have a primary thallus and a secondary thallus. The primary thallus is small and clings closely to the substrate while the secondary thallus is a shrubby growth like fruticose lichens.