Rise of the Animal Kingdom and Epigenetic Mechanisms of Evolution - Building the Most Complex Structure on Earth - page 272

MRL mouse—ear lobe regeneration
Ear base

Ear tip

Normal Denervated

(A)

(B)

Day 0

Distal

Proximal

Day 50

Nerve bundles

Blastema

Epithelial downgrowth

Figure 5.9 Innervation and mammalian regeneration. Regeneration of the pinna in the MRL
mouse. (A) Injury induced by an ear punch is completely healed by day 50 in the presence
of normal innervation. Transection of the nerves to the ear lobe prior to ear punch results
in abnormal wound healing and necrosis of the tissue. (B) Diagrammatic representation of
a regenerating ear-punch hole. Quantitative image analysis has shown that the density of
innervation is higher in the proximal ear base of the pinna compared to the distal tip.
Source : Kumar and Brockes (2012) .

CNS and the structural complexity, as it manifests in the evolution of specialized ani-
mal organs and structures, through the course of the evolution of bilaterians.

Acoela (Acoelomates)
Acoela are the most primitive group of extant bilaterians with a rudimentary centralized
nervous system. The group consists of 340 species and 21 families known so far ( Hooge
and Tyler, 2006 ). Their nervous system shows only a minute anterior-posterior gradient
of neuronal concentration ( Semmler et al., 2010 ), which is far from a “compact brain.”
The nervous system consists of one or more commissural rings in the anterior part that
are connected with longitudinal nerve cords along the length of the body; hence, they
are termed “orthogon” for their nervous system and the designation “commissural
brain” for the anterior concentration of neurons (Raikova et al., 2001) ( Figure 5.10 ).
Acoela belong to the phylum Acoelomorpha and are believed to be the earli-
est offshoot of bilaterians ( Bery et al., 2010; Jondelius et al., 2002 ; Peterson et al.,
2008). They tend to be free-living unsegmented marine animals that lack a coelom
(body cavity), have no anus and no appendages, and move via the ciliary epithelium
on their ventral surface.
Substantial evolutionary progress is observed in flatworms ( Platyhelminthes ). The
progress in the organization and centralization of the nervous system is reflected in
the evolution of the excretory and digestive systems. Flatworms lack circulatory and
pulmonary systems, but because of their small body sizes their cells are close enough
to nutrients and oxygen.
Recent studies in the planarian flatworm, Schmidtea mediterranea , have identi-
fied more than 140 neuropeptides and peptide hormones revealing the complexity of
the nervous system of S. meditteranea , one of the most primitive centralized nervous
systems in invertebrates. Many of these are neurohormones secreted only by neurons