Rise of the Animal Kingdom and Epigenetic Mechanisms of Evolution - Building the Most Complex Structure on Earth - page 273

(A) (B)

In2
In1
mn

In2
In1

mn

Figure 5.10 Semi-schematic sketch drawing of the juvenile (A) and the adult (B)
serotonergic nervous system of the acoelomate Symsagittifera roscoffensis . Six longitudinal
neurite bundles extend along the anterior-posterior body axis: two median (mn) and four
lateral (ln1 and ln2) ones.
Source : Semmler et al. (2010) .

in the CNS. Chemical signals, especially members of the NPY-8 family of neurohor-
mones released by the cephalic ganglia, are essential for the development and main-
tenance of reproductive organs. In S. mediterranea :

peptides (e.g., NPY-8 peptides) from the nervous system promote events associated
with reproductive maturation (i.e., the production of differentiated germ cells)
Collins et al. (2010)

Evolution of the “True” Brain
The true compact brain in the anterior of the animal body is believed to have
emerged four times, thrice in protostomes (i.e., arthropods, mollusks, and annelids)
and once in the latest common ancestor of cephalochordates, urochordates, and ver-
tebrates ( Figure 5.11 ).

Annelids
The earliest annelid fossil comes from the lower to middle Adtabanian, about
520 Mya (Conway Morris and Peel, 2008).