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sponges, appear in the fossil record about 580 Mya ( Li et al., 1998 ). They have a
system of canals through which water circulates bringing food and oxygen to cells in
the interior of the body. Most cells maintain a relatively high independence and the
number of cell types is low, about 7-8. Sponges have no precise form and symmetry
as eumetazoans do. They also lack true tissues and organs ( Ereskovsky and Dondua,
2006 ). A sponge's structure and functions at the supracellular level are modest and
its control system is barely discernible.
Sponges are a “dead end of evolution”; there is no evidence that they, or other
simple creatures, such as placozoans, evolved into any higher groups of extant or
extinct metazoans ( Figure 5.6 ).
Why did sponges fail to evolve into other metazoan groups of higher structural
and functional complexity as the eumetazoans did?
Any comparison of the differences between sponges and their possible sis-
ter group, cnidarians, the simplest of known eumetazoans, reveals that sponges are
genetically not inferior; the sponge, Amphimedon queenslandica , genome encodes
Deuterostomes
Chordates
Urochordates
Cephalochordates
Heomichordates
Echinoderms
Protostomes
Annelids
Molluscs
Nemertines
Platyhelminthes
Arthropods
Priapulides
Nematodes
Coelenterates
Cnidaria
Medusozoa
Anthozoa
Ctenophora
CNS
glial cells
Porifera
Placozoa
Ur-Bilateria: last common
bilaterian ancestor ~540 Mya
Nerve rings, sensory organs (eyes), muscles
Nerve cells, nerve nets, myoepithelial cells
Ur-Eumetazoa: last common
eumetazoan ancestor
Ur-Metazoa: last common
metazona ancestor ~600 Mya
Sensory-
effector cells
Choanoflagellates
Figure 5.6 Tree representing the main steps in the formation of first-evolved nervous
systems along the animal phylogeny (blue branches). This tree was deduced from the cellular
and physiological analyses performed in choanoflagellates that behave as sensory-effector
cells, in Porifera (sponges) that lack synaptic transmission but exhibit contractile behavior in
response to their environment, and in coelenterates (grouping Ctenophora—comb jellies—and
Cnidaria). Coelenterates show coordinated behaviors thanks to their nervous systems that
already include sensory organs, nerve rings, and neuromuscular transmission. Bilaterians
(protostomes and deuterostomes) that originated later share a putative common ancestor
(Urbilateria) equipped with a CNS and differentiating glial cells.
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