trace the different variations between nervous systems in the animal kingdom?

Answer 1

There is no simple answer to this question, but sounds like it could be a good research topic. Here are some tid bits that I recall from way back: The coelenterates (like the hydra & jellyfish) have a "nerve net" ; no centrally located nervous system, therefore impulses move in no set pattern of direction.
As you move up the phylogenetic tree, the nervous systems centralize, as in the worms (like the flatworms;planaria). An advanced worm, like the earthworm or leech (an annelid)
have a central nervous system with a primitive brain
(actually, a pair of fused ganglia). The insects (arthropods)
have a similar central nervous system that is more developed because these animals are more complex (have legs, wings, eyes). Note that the location of the central nervous system in Invertebrates is along the ventral ("belly")
side of the animal. A centrally located NS gives the animal good directions for the flow of impulses and therefore coordination. The most advanced vertebrates have a well defined central NS, located on the dorsal (back) side, with a well defined , complex brain at the anterior (head) part of the body.