The Elementary Nervous System Revisited

Parker's theory of the origin of the nervous system is discussedalong with later interpretations. Attention today has shiftedfrom the cellular to the molecular level, and it has becomeclear that many of the molecules and mechanisms thought of astypically neuronal have homologs or counterparts in non-nervouscells and unicellular organisms. This applies to signallingchemicals, receptors, second messenger systems and ion channels,and also to the production of electrical events. Parker's viewof sponges as a group lacking nerves but possessing independenteffectors is still acceptable, but some sponges (and also higheranimals) employ non-nervous signalling pathways to coordinatetheir effectors. Thus, nerves are not always necessary for coordinatedbehavior. Cnidarians like hydra have seemingly simple, two-dimensionalnervous systems with little or no centralization, but even suchsystems can be surprisingly complex, and the more advanced cnidariansshow neurophysiological specializations as sophisticated asthose of many higher invertebrates. Examples of ingenious cnidariansolutions to behavioral problems are given. No existing animalshave ‘elementary’ nervous systems if that term impliesthe existence of crude or inefficient functional adaptations