Revisiting nerve nets: functional organization and evolutionary implications.

Nerve nets-characterised by scattered neurons and mesh-like projections-represent a widespread organisation of nervous tissue. Nerve nets have uniquely diffuse conductivity and an apparent simplicity that features in many theories on the evolution of nervous systems. Here we review the anatomical and functional organisation of nerve nets and revisit evolutionary theories in light of new information. We discuss how transgenesis and volume electron microscopy begin to reveal the fine-scale cellular organisation of nerve nets. On the functional level, we describe how excitation can spread within nerve nets and how non-synaptic volume transmission enable multi-layered signalling, driving complex behaviours. As a convergent solution to tissue-scale coordination, we introduce ctenophores, where syncytial nerve nets implement distributed control. We argue that nerve nets are not primitive remnants in some animals or only historical preludes but represent an enduring organisational principle for animal coordination with many unique advantages.