Cephalization is an
evolutionIn biology, evolution is change in the genetic material of a population of organisms from one generation to the next. Though changes produced in any one generation are normally small, differences accumulate with each generation and can, over time, cause substantial changes in the population, a...
ary trend, whereby
nervous tissueThe nervous system is a network of specialized cells that communicate information about an organism's surroundings and itself. It processes this information and causes reactions in other parts of the body. It is composed of neurons and other specialized cells called glial cells that aid in the...
, over many generations, becomes concentrated toward one end of an organism. This process eventually produces a
headIn anatomy, the head of an animal is the rostral part that usually comprises the brain, eyes, ears, nose, and mouth...
region with
sensory organsA sensory system is a part of the nervous system responsible for processing sensory information. A sensory system consists of sensory receptors, neural pathways, and parts of the brain involved in sensory perception. Commonly recognized sensory systems are those for vision, hearing, somatic...
.
Cephalization is intrinsically connected with a change in
symmetrySymmetry in biology is the balanced distribution of duplicate body parts or shapes. The body plans of most multicellular organisms exhibit some form of symmetry, either radial symmetry or bilateral symmetry or "spherical symmetry". A small minority exhibit no symmetry .In nature and biology,...
. It accompanied the move to bilateral symmetry made in
flatwormThe flatworms, known in scientific literature as Platyhelminthes are a phylum of relatively simple bilaterian, unsegmented, soft-bodied invertebrate animals...
s, with ocelli and 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
annelidThe annelids, collectively called Annelida , are a large phylum of segmented worms, with over 17,000 modern species including ragworms, earthworms and leeches. They are found in marine environments from tidal zones to hydrothermal vents, in freshwater, and in moist terrestrial environments...
s on also place the mouth in the head region. This process is also tied to the development of an anterior
brainThe brain is the center of the nervous system in all vertebrate, and most invertebrate, animals. Some primitive animals such as jellyfish and starfish have a decentralized nervous system without a brain, while sponges lack any nervous system at all...
in the chordates from the
notochordThe notochord is a flexible, rod-shaped body found in embryos of all chordates. It is composed of cells derived from the mesoderm and defines the primitive axis of the embryo. In some chordates, it persists throughout life as the main axial support of the body, while in most vertebrates it is...
. A notable exception to the trend of cephalization throughout evolutionary advancement is
phylumIn biology, a phylum ["Phylum" is adopted from the Greek phylai, the clan-based voting groups in Greek city-states.] is a taxonomic rank below Kingdom and above Class...
EchinodermEchinoderm, there are seven main classes of Echinoderms which are brittle stars, basket stars, sea urchins, sand dollars, sea lilies, feather stars, and sea cucumbers...
ata, which, although having a bilateral ancestor, as evidenced by their
embryologyEmbryology is the study of the development of an embryo. An embryo is defined as any organism in an early stage well before birth or hatching, or in plants, before germination occurs....
, 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
ectodermThe ectoderm is the start of a tissue that covers the body surfaces. It emerges first and forms from the outermost of the germ layers.Generally speaking, the ectoderm differentiates to form the nervous system, and the epidermis .In vertebrates, the ectoderm has three parts: external ectoderm , the ...
forms a
neural tubeIn the developing vertebrate, the neural tube is the embryo's precursor to the central nervous system, which comprises the brain and spinal cord...
which undergoes cephalization to form initially 3, then 5
vesiclesA vesicle is a bubble of liquid within a cell. More technically, a vesicle is a small, intracellular, membrane-enclosed sac that stores or transports substances within a cell. Vesicles form naturally because of the properties of lipid membranes ...
as a developing embryo. It is the internalized ectoderm which goes on to become the
central nervous systemThe central nervous system is the part of the nervous system that functions to coordinate the activity of all parts of the bodies of bilaterian animals—that is, all animals more advanced than sponges or jellyfish. In vertebrates, the central nervous system is enclosed in the meninges. It contains...
,
peripheral nervous systemThe peripheral nervous system resides or extends outside the central nervous system , which consists of the brain and spinal cord. The main function of the PNS is to connect the CNS to the limbs and organs. Unlike the central nervous system, the PNS is not protected by bone or by the blood-brain...
and
epidermisThe epidermis is the outer layer of the skin, composed of terminally differentiated stratified squamous epithelium, acting as the body's major barrier against an inhospitable environment...
.
In respirology, cephalization refers to the distribution of pulmonary flow from the bases to the apices of the
lungThe lung or pulmonary system is the essential respiration organ in air-breathing animals, including most tetrapods, a few fish and a few snails. In mammals and the more complex life forms, the two lungs are located in the chest on either side of the heart...
due to
pulmonary edemaPulmonary edema , or oedema , is fluid accumulation in the lungs. It leads to impaired gas exchange and may cause respiratory failure. It is due to either failure of the heart to remove fluid from the lung circulation or a direct injury to the lung parenchyma...
.