Meinertzhagen, I.A. 2010. The organisation of invertebrate brains: cells, synapses and circuits. —Acta Zoologica (Stockholm) 91: 64–71
Invertebrate brains are structurally diverse. Neuron numbers range from ∼102 to 108 in different groups, compared with larger numbers in vertebrate brains, ∼107 to 1014. The underpopulated brains of invertebrates are noted in their extreme cases for having few cells, and neurons that can be identified from animal to animal, many known in great detail. Although few in number, invertebrate neurons nevertheless comprise many classes. Correlated with the paucity of their number they are sparsely connected, many having ∼50 synapses or fewer. Synaptic densities, roughly 1 per μm3 of neuropile, differ little from those for much larger vertebrate neurons. Invertebrate neurons differ from their vertebrate counterparts in the position of their soma, generally in a cortex surrounding the neuropile that consequently occupies a relatively small volume. Their axons typically lack myelin and, supporting a range of conduction velocities, have diameters that differ over a wide range, from 103 to 10−1μm. Nerves with thousands of axons differ from neuropile fascicles, which typically have 20 or less. Unlike most vertebrate synapses, but like those of the vertebrate retina, synapses in many invertebrate groups – probably all ecdysozoans and possibly some lophotrochozoans – have synaptic contacts with multiple postsynaptic elements, dyads, triads and so on.