Cephalization is an evolutionary trend towards the development of a brain and head. This trait should seem familiar, since humans exemplify it. This trend does offer organisms several advantages over other body shapes in some situations. Like all evolutionary trends, it happens at the species level, not at the individual level. Cephalization ties in with several other evolutionary trends, including bilateral symmetry. Humans share these characteristics with many other animal phyla.
Cephalization has several components. First, includes the development of a recognizable front end, differentiated from the back end. This process also involves concentrating sensory organs at the front of an organism, as well as feeding organs like a mouth and jaws, and nerve tissue. In some organisms, this nerve tissue gets more complicated, leading to ganglia -- a clump of nerves -- and eventually a brain. An animal with a well-developed head can be described as "highly cephalized." You can summarizes all of this by saying that cephalization as the process of a phyla developing a defined head. It is important to note that cephalization, like all evolutionary action, happens above the individual level. Individuals do not undergo cephalization, but species do.
Several different animal groups have undergone cephalization separately, which indicates that it offers some advantages over other body types. For this reason, it can be tempting to think of cephalization as a kind of "progress." However, this is very human-centered thinking, since evolution has no direction or goal. Many phyla have never under undergone cephalization. For example, echinoderms -- the phyla that includes starfish and sea urchins -- have never undergone cephalization. Other groups, like some parasitic animals, have actually "lost" their heads, since they had little use for them inside of other organisms.
A species cannot undergo cephalization unless it first goes through several evolutionary trends. First, an species has to develop bilateral symmetry. Organisms with bilateral symmetry have a distinct right and left side. This contrasts with asymmetry and radial symmetry. In organisms with radial symmetry, you could take cross-sections of the organism across the center from multiple angles and get identical pieces. One way to help visualize this is to remember that a hot dog in a bun has bilateral symmetry, while a hamburger has radial symmetry. Bilateral symmetry sets a species up for cephalization, since organisms with bilateral symmetry can now have a "front" end. Since the front of an organism is the first part to enter a new area, concentration feeding and sensory structures at the front gives it an adaptive advantage.
Examples of Cephalization
Cephalization is widespread across the animal kingdom. You can find many examples of this trend from different phyla, including both invertebrates and vertebrates. This includes examples in the arthropods, mollusks, annelids, and all vertebrates. Many arthropods, including crustaceans, insects and arachnids have cephalization. Some mollusks have high degrees of cephalization like the cephalopod. Cephalopods includes squid, octopuses and related mollusks. Additionally, the annelids, better known as segmented worms, have cephalization. Additionally, all chordates have cephalization. This means every organism with a spinal cord or backbone, including humans, have a high degree of cephalization.
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