Rise of the Animal Kingdom and Epigenetic Mechanisms of Evolution - Building the Most Complex Structure on Earth - page 274

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Figure 5.11 The last bilaterian common ancestor possessed a diffuse nerve plexus and
brains independently evolved at least four times among bilaterians.
Source : Northcutt (2012) .

The annelid nervous system consists of a primitive compact brain in the anterior
of the body connected with two ventral nerve cords that connect with ganglia in each
segment. Annelids have evolved specialized sense organs (eyes, taste buds, stato-
cysts, etc.) and a relatively complex neuroendocrine system that comprises secre-
tory neurons with projections throughout the worm body. They secrete more than 40
neuropeptides and neurohormones that circulate in hemolymph. Most are mamma-
lian-like neuropeptides, such as angiotensin-like peptides, oxytocin/vasopressin, and
enkephalins ( Salzet, 2007; Veenstra, 2011 ).
Morphologically, annelids display a great progress in comparison to flatworms;
their digestive system, besides mouth and guts, also has an anus. The body is seg-
mented with a cavity, although it is not a true coelom. They have evolved a closed