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Bilateral brain
(A)
Segmental
ganglia
(D)
(C)
Nerve
network
(B)
Anterior
ganglia
Hydra
Nerve
trunks
Earthworm
Planaria
Grasshopper
Figure 5.8
Invertebrate nervous systems. (A) Nerve net of radiates, the simplest neural
organization. (B) Flatworm system, the simplest linear-type nervous system of two nerves
connected to a complex neuronal network. (C) Annelid nervous system, organized into a
bilobed brain and ventral cord with segmental ganglia. (D) Arthropod nervous system with
large ganglia and more elaborate sense organs.
Source
: Biocyclopedia. Available from:
http://www.biocyclopedia.com/index/general_
in diploblastic animals (cnidarian and ctenophores). There is no hint that triploblasty
is related to bilaterality neither to the evolution of the nervous system, which devel-
ops from the ectoderm, nor to cephalization.
Bilateral symmetry is a property or geometric concept, not a functioning struc-
ture; it is an effect of development rather than a causal agent. Hence, there is no way
for it to induce the formation of organs, a nervous system, or cephalization.
Evolution of cephalization is related to the evolution of the CNS, but it is causally
not related to the evolution of organs and organ systems or to bilaterality. When it
comes to the evolution of the CNS, the temporal coincidence with all the aforemen-
tioned structures and qualities displays a clear causal relationship. Signals and cel-
lular elements from the CNS determine or participate in the formation of all organs,
cephalization, and the development of bilateral symmetry (see Chapter 3,
Epigenetic
Control of Animal Development
).
None of the aforementioned, individually or collectively, proves that the central-
ized nervous system determines the bilateral symmetry in bilaterians. Now, half a
billion years after the Cambrian, we possess a wealth of fossil evidence on the
astounding radiation of animal phyla, but we are left with little means of determining
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