Editing Cephalization (section)

== Partly cephalized phyla ==

[[File:Yellow papillae flatworm (Thysanozoon nigropapillosum) (cropped) (cropped).jpg|thumb|upright=1.15|The gold-speckled flatworm, ''[[Thysanozoon nigropapillosum]]'', is somewhat cephalized, with a distinct head end (at right) which has pseudotentacles and an photoreceptive eyespot.]]

The [[Acoela]] are basal bilaterians, part of the [[Xenacoelomorpha]]. They are small and simple animals with flat bodies. They have slightly more nerve cells at the head end than elsewhere, not forming a distinct and compact brain. This represents an early stage in cephalization.<ref name="Cabej2013">{{cite book |last=Çabej |first=Nelson |chapter=Rise of the Animal Kingdom and Epigenetic Mechanisms of Evolution |title=Building the most complex structure on Earth: an epigenetic narrative of development and evolution of animals |publisher=[[Elsevier]] |year=2013 |isbn=978-0-12-401667-5 |pages=239–298|doi=10.1016/B978-0-12-401667-5.00005-5}}</ref>

Also among the bilaterians, [[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.<ref name="Cabej2013"/>

Among animals without bilateral symmetry, the [[Cnidaria]], such as the radially symmetrical (roughly cylindrical) [[Hydrozoa]], show some degree of cephalization. The [[Anthomedusae]] have a head end with their mouth, [[photoreceptor cell]]s, and a concentration of nerve cells.<ref>{{cite book |last1=Satterlie |first1=Richard |editor1-first=John H |editor1-last=Byrne |title=The Oxford Handbook of Invertebrate Neurobiology |chapter=Cnidarian Neurobiology |volume=1 |pages=184–218 |publisher=Oxford University Press |doi=10.1093/oxfordhb/9780190456757.013.7 |date=February 2017 |quote=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). |isbn=9780190456757}}</ref>