n
body plan. With a cylindrical body (in the main clade, the
nephrozoa) and a direction of
travel, the animal has head and tail ends, favouring cephalization by
natural selection. Sense organs, brain, and mouth form the basis of the head. These have the ability to move, using muscles, and a
body plan with a front end that encounters stimuli first as the animal moves forwards, and accordingly has evolved to contain many of the body's sense organs, able to detect light, chemicals, and gravity. There is often a collection of nerve cells able to process the information from these sense organs, forming a
brain in several
phyla and one or more
ganglia (clusters of nerve cells) in others.
Complex active bodies The philosopher Michael Trestman noted that three bilaterian phyla, namely the arthropods, the molluscs in the shape of the cephalopods, and the chordates, were distinctive in having "complex active bodies", something that the acoels and flatworms did not have. Any such animal, whether predator or prey, has to be aware of its environment—to catch its prey, or to evade its predators. These groups are exactly those that are most highly cephalized. These groups, however, are not closely related: in fact, they represent widely separated branches of the Bilateria, as shown on the
phylogenetic tree; their lineages split hundreds of millions of years ago. Other (less cephalized) phyla are omitted for clarity. }}
Arthropods In
arthropods, cephalization progressed with the gradual incorporation of trunk segments into the head region. This was advantageous because it allowed for the evolution of more effective mouth-parts for capturing and processing food. Insects are strongly cephalized, their brain made of three fused
ganglia attached to the
ventral nerve cord, which in turn has a pair of ganglia in each segment of the
thorax and
abdomen, the parts of the trunk behind the head. The
insect head is an elaborate structure made of several
segments fused rigidly together, and equipped with both simple and
compound eyes, and multiple
appendages including sensory
antennae and complex
mouthparts (maxillae and mandibles). They are highly cephalized, with well-developed senses, including
advanced 'camera' eyes and large brains.
Vertebrates Cephalization in
vertebrates, the group that includes
mammals,
birds,
reptiles,
amphibians and
fishes, has been studied extensively. and a large, multi-lobed brain protected by a skull of bone or
cartilage. In the 1980s, the
new head hypothesis proposed that the vertebrate head is an evolutionary novelty resulting from the emergence of
neural crest and cranial
placodes (thickened areas of the embryonic
ectoderm layer), which result in the formation of all sense organs outside the brain. However, in 2014, a transient
larva tissue of the lancelet was found to be virtually indistinguishable from the
neural crest-derived cartilage (which becomes
bone in jawed animals) which forms the vertebrate
skull, suggesting that persistence of this tissue and expansion into the entire head space could be a viable evolutionary route to forming the vertebrate head. Advanced vertebrates have increasingly elaborate brains. == Partly cephalized phyla ==