Figure 1. Different types of invertebrate nervous systems. (a) A simple nerve net of loosely associated neurons in a sponge connected at points of contact by synapses (high magnification view). (b) The ladder-like layout of a flatworm, with two longitudinal connectives running the length of the body, joined at intervals by commissures. (c) The segmentally organized nervous system of an annelid, with the longitudinal connectives and ganglia fused along the midline. There is an elongated ganglion for each segment. (d) Three ganglia located in the segmented thorax of an insect. The ‘metathoracic ganglion’ is really a fused structure containing neurons of the metathoracic ganglion and the first three abdominal ganglia.

Figure 2. Cross-section through an insect ganglion to show longitudinal bundles (tracts) of axons, neuron somata in a layer around the margin, commissures containing branches of neurons that cross the midline, and the remaining areas of neuropil where almost all of the fine branches and synapses are found. In a large insect like a locust, a ganglion measures about 1 mm across.