Anthomedusae have a head end with their mouth, photoreceptive cells, and a concentration of neural cells.[1]
Bilateria
Idealised bilaterianbody plan. With a cylindrical body and a direction of forward movement the animal has head and tail ends, favouring cephalization by natural selection. Sense organs and mouth form the basis of the head.
bilaterians, a large group containing the majority of animal phyla.[2] 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 also 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 in others.[3]
Acoela
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]
Flatworms
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]
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.[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]
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]
Brains of shark and human illustrate accelerating cephalization, as the parts enlarge and change shape.
Cephalization in
placodes (thickened areas of ectoderm), which result in the formation of all senses outside of the brain.[12][13] However, in 2014, a transient larva tissue of the lancelet was found to be virtually indistinguishable from the neural crest-derived cartilage (later bone, in jawed ones) 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]
. Ocelli located at the base of the many tentacles represent one input to the B system, whereas the neurons of the O system are directly photosensitive. Many hydromedusae have ocelli of different levels of complexity (Singla, 1974). In addition, other marginal sensory structures associated with the outer nerve ring include statocysts (Singla, 1975), and mechanoreceptors, such as the tactile combs of Aglantha, which are located at the tentacle bases, and can activate the escape swimming circuitry (Arkett & Mackie, 1988; Mackie, 2004b).
_(cropped).jpg)
.jpg){kind=small}
.png){kind=small}
.jpg){kind=small}
