Cephalization

The Acoela are basal bilaterians, part of the Xenacoelomorpha. They are small and simple animals, and have very slightly more nerve cells at the head end than elsewhere, not forming a distinct and compact brain. This represents an early stage in cephalization.[4]

The yellow papillae flatworm, Thysanozoon nigropapillosum, is somewhat cephalized, with a distinct head end (at right) which has pseudotentacles and an eyespot.

The Platyhelminthes (flatworms) have a more complex nervous system than the Acoela, and are lightly cephalized, for instance having an eyespot above the brain, near the front end.[4]

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.[5][6] 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 not shown, for clarity.[7][8][9]

In arthropods, cephalization progressed with increasing 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 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).[4]

Cephalopods like this cuttlefish have advanced 'camera' eyes.

Cephalopod molluscs including octopus, squid, cuttlefish and nautilus are the most intelligent and highly cephalized invertebrates, with well-developed senses, including advanced 'camera' eyes and large brains.[10][11]

Brains of shark and human illustrate accelerating cephalization, as the parts enlarge and change shape.

Cephalization in vertebrates, the group that includes mammals, birds, and fishes, has been studied extensively.[4] The heads of vertebrates are complex structures, with distinct sense organs; a large, multi-lobed brain; jaws, often with teeth; and a tongue. Cephalochordates like Branchiostoma (the lancelet, a small fishlike animal with very little cephalization), are closely related to vertebrates but do not have these structures. 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 ectoderm).[12][13] However, in 2014, a transient larva tissue of the lancelet was found to be virtually indistinguishable from the neural crest-derived cartilage 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 formation of the vertebrate head.[14] Advanced vertebrates have increasingly elaborate brains.[4]