Editing Nervous system (section)

====Neural precursors in sponges====
[[Sponge]]s have no cells connected to each other by [[synapse|synaptic junctions]], that is, no neurons, and therefore no nervous system. They do, however, have [[homology (biology)|homologs]] of many genes that play key roles in synaptic function. Recent studies have shown that sponge cells express a group of proteins that cluster together to form a structure resembling a [[postsynaptic density]] (the signal-receiving part of a synapse).<ref name=Sakarya/> However, the function of this structure is currently unclear. Although sponge cells do not show synaptic transmission, they do communicate with each other via calcium waves and other impulses, which mediate some simple actions such as whole-body contraction.<ref name=Jacobs/>