Table 1.

The ‘explanatory model’ and ‘function’ matrix of the origin of nervous systems. The entries in the cells give examples, in simple organisms, of each of the six roles that nervous systems can play.

	function
explanatory models	neural control of behaviour	neural control of physiology	neural control of development
input–output	phototaxis; chemotaxis; toxin gland discharge	light-controlled reproduction; salivary reflex	environmentally triggered metamorphosis
internal coordination	jet propulsion; undulatory swimming	gut peristalsis; diuresis control; neuroendocrine control of metabolism	neuroendocrine control of growth; ecdysis