Nervous System

The earthworm's nervous system is centred on a bilobed mass of nerve tissue on the dorsal side of the pharynx in segment 3, functionally equivalent to a brain and called the supra-pharyngeal ganglion. A second ganglion lying beneath the pharynx in segment 4, the sub-pharyngeal ganglion, acts with it; a pair of connectives running on either side of the pharynx link the two ganglia into a complete nerve ring encircling the front of the gut. From the sub-pharyngeal ganglion, a double ventral nerve cord runs backward through the length of the body, carrying signals to and from every segment. This system integrates input from several kinds of receptors distributed over the body - photoreceptors on the dorsal surface for light, gustatory and olfactory receptors in the buccal cavity for taste and smell, and tactile, chemo- and thermoreceptors in the prostomium an …