They lack sensory organs/neurons derived from the olfactory placode (olfactory epithelium, vomeronasal organ and gonadotropin-releasing hormone neurons) and the otic placode (cochlea, vestibule and vestibuloacoustic ganglion), and show reduced neural differentiation in the distal portion of the epibranchial ganglia derived from the epibranchial placodes [5 –10 ].