Cephalization 1065888 215147095 2008-05-26T22:31:50Z 67.175.22.101 {{Unreferenced|date=February 2007}} [[Image:Tenia_solium_scolex.jpg|thumb|200px|''Tenia solium'', a [[cestoda|cestode]] showing simple cephalization.]] '''Cephalization''' is an [[evolution]]ary trend, whereby [[nervous system|nervous tissue]], over many generations, becomes concentrated toward one end of an organism. This process eventually produces a [[head]] region with [[sensory system|sensory organs]].<ref>Biology/Neil A. Campbell, Jane B. Reece.-7th Edition (AP Edition) </ref> Cephalization is intrinsically connected with a change in [[symmetry (biology)|symmetry]]. It accompanied the move to bilateral symmetry made in [[flatworm]]s, with [[ocelli]] and [[pinna (anatomy)|pinnae]] placed in the head region. The cephalization/bilateral symmetry combination allowed animals to have sensory organs facing the direction of movement, allowing a more focused assessment of the environment into which they are moving. In addition to a concentration of sense organs, all animals from [[annelid]]s on also place the mouth in the head region. This process is also tied to the development of an anterior [[brain]] in the [[chordata|chordates]] from the [[notochord]]. A notable exception to the trend of cephalization throughout evolutionary advancement is [[phylum]] [[Echinoderm|Echinodermata]], which, although having a bilateral ancestor, as evidenced by their [[embryology]], develop into a pentaradial animal with no concentrated neural ganglia or sensory head region. However, some echinoderms have developed bilateral symmetry secondarily. In [[neuroembryology]], neural induction of the [[ectoderm]] forms a [[neural tube]] which undergoes cephalization to form initially 3, then 5 [[vesicle (biology)|vesicles]] as a developing embryo. It is the internalized ectoderm which goes on to become the [[central nervous system]], [[peripheral nervous system]] and [[epidermis (skin)|epidermis]]. [[Category:Evolutionary biology|Cephalization]] [[Category:Evolution|Cephalization]] [[es:Cefalización]] [[fr:Céphalisation]]