Annelids, like the earthworm, and arthropods, like insects or spiders, have three main structures in their nervous systems:
-a brain, a dorsal nerve cord, and a ventral nerve cord
-a brain, segmented ganglia, and a ventral nerve cord
-segmented ganglia, a dorsal nerve cord, and a ventral nerve cord

The answer is Segmented ganglia, a dorsal nerve cord and a ventral nerve cord.

Explanation:

In case of invertebrates like annelids and arthropods the nervous system is not so well developed. The nerve cells are not organized into brain but they have collection of nerve cells which makes ganglia. Annelids are both externally and internally segmented whereas arthropods show only external segmentation. The presence of ganglia in each segment gives the name segmented ganglia. Along with this they have nerve cord running from anterior to posterior end. The one present on the dorsal side is called dorsal nerve cord and the one present on the ventral side is called ventral nerve cord.

in 1859, charles darwin set out his theory of evolution by natural selection as an explanation for adaptation and speciation. he defined natural selection as the "principle by which each slight variation [of a trait], if useful, is preserved".