The definition of Cephalization is the development of the head. Paedomorphsis and cephalization It is the tendency for sensory, neural, and feeding organs to be concentrated at the front end of the body, leading to the development of a head during both evolution and embryological development. It is considered an evolutionary trend whereby nervous tissue, over many generations, becomes concentrated toward one end of the organism. This process eventually produces a head region with sensory organs. It is intrinsically connected with a change in symmetry. It accompanied the move to bilateral symmetry made in flatworms, with ocelli and pinnae placed in the head region. In addition to a concentration of sense organs, all animals from annelids on also place the mouth in the head region. This process is also tied to the development of an anterior brain in the chordates from the notochord. A notable exception to the trend of cephalization throughout evolutionary advancement is phylum Echinodermata, which although having a bilateral ancestor, as evidenced by their embryology, develop into a pent radial animal with no concentrated neural ganglia or sensory head region. However, some echinoderms have developed bilateral symmetry secondarily.
Flatworms (phylum Platyhelminthes) are the most primitive animals with bilateral symmetry. They also have a fairly advanced degree of cephalization, with sense organs (photo sensory and chemosensory cells) and brain concentrated at the anterior end. Consequently, scientists believe that cephalization characterized all bilaterally symmetrical animals from their origins. However, flatworms differ from more advanced animals in that their mouths are in the center of their bodies, not at the anterior end. In segments into the head region. Scientists believe this was advantageous because it allowed for the evolution of more effective mouth-parts for capturing and processing food.
For cephalochordate Branchiostoma (formerly called Armphioxus), which is the closest relative of vertebrates, is a burrowing marine creature which lacks most of the head structures that are so distinct in vetebrates, such as distinct sense organs; a large multilobed brain; teeth; and a tongue. There was a persistent debate during the twentieth century as to whether the vertebrates head is “old” or “new”. Scientists who champion the idea of the “old” head suggest the vertebrate head resulted from the evolution of important modifications to a previously existing head. The idea of a “new” vertebrate head was proposed originally by American vertebrate head is a new structure, which has on corresponding structure in close relatives such as Branchiostoma. Evidence to support a “new” vertebrate head come sfrom the observation that most important features of the head are derived from neutral crest cells, embryonic cells found only in vertebrate.
Consequently, scientists believe that cephalization characterized all bilaterally symmetrical animals from their origins. However, flatworms differ from more advanced animals in that their mouths are in the center of their bodies, not at the anterior end. Important features of the head are derived from neural crest cells, embryonic cells found only in vertebrates. The neural crest cells are of ectodermal origin rather than mesodermal or endodermal and arise during the process of neurulation, the time at which the dorsal hollow nerve cord forms.
Paedomorphosis is the retention by an organism of juvenile or even larval traits into later life. Paedomorphosis is the evolutionary retention of larval traits in an adult body. Some occur in some of the amphibians. There are two aspects of Paedomorphosis: