Fig. 10

A possible scenario of the evolution of the cerebral ganglion. According to this scenario, a hypothetical neuroepithelium evolved into a concave ganglion in cyclostomates, and then into a ganglion with a small microvilli-filled lumen in ctenostomates, and finally into a ganglion with a prominent lumen in phylactolaemates. In cyclostomes, the ganglion retains cilia and microvilli; in ctenostomes, the ganglion lacks cilia but retains the basal bodies of the cilia and microvilli; and in phylactolaemates, the ganglion lacks both microvilli and cilia. In all cases, cells of the ganglion are connected by desmosomes. Abbreviations: ac – accessory centriole; bb – basal body; bl – basal lamina; ccg – concavity above the cerebral ganglion; ci – cilia; de – desmosome; lcg – lumen of the cerebral ganglion; mi – microvilli; sv – synaptic vesicles