Cephalization: Definition and Examples

In zoology, cephalization is the evolutionary trend toward concentrating nervous tissue, the mouth, and sense organs toward the front end of an animal. Fully cephalized organisms have a head and brain, while less cephalized animals display one or more regions of nervous tissue. Cephalization is associated with bilateral symmetry and movement with the head facing forward.

Advantages

Cephalization offers an organism three advantages. First, it allows for the development of a brain. The brain acts as a control center to organize and control sensory information. Over time, animals can evolve complex neural systems and develop higher intelligence. The second advantage of cephalization is that sense organs can cluster at the front of the body. This helps a forward-facing organism efficiently scan its environment so it can locate food and shelter and avoid predators and other dangers. Basically, the front end of the animal senses stimuli first, as the organism moves forward. Third, cephalization trends toward placing the mouth closer to the sense organs and brain. The net effect is that an animal can quickly analyze food sources. Predators often have special sense organs near the oral cavity to gain information about prey when it's too close for vision and hearing. For example, cats have vibrissae (whiskers) that sense prey in the dark and when it's too close for them to see. Sharks have electroreceptors called the ampullae of Lorenzini that allow them to map prey location.

Examples of Cephalization

Three groups of animals display a high degree of cephalization: vertebrates, arthropods, and cephalopod mollusks. Examples of vertebrates include humans, snakes, and birds. Examples of arthropods include lobsters, ants, and spiders. Examples of cephalopods include octopuses, squid, and cuttlefish. Animals from these three groups exhibit bilateral symmetry, forward movement, and well-developed brains. Species from these three groups are considered to be the most intelligent organisms on the planet.

Many more types of animals lack true brains but have cerebral ganglia. While the "head" may be less clearly defined, it's easy to identify the front and rear of the creature. Sense organs or sensory tissue and the mouth or oral cavity is near the front. Locomotion places the cluster of nervous tissue, sense organs, and mouth toward the front. While the nervous system of these animals is less centralized, associative learning still occurs. Snails, flatworms, and nematodes are examples of organisms with a lesser degree of cephalization.

Animals That Lack Cephalization

Cephalization doesn't offer an advantage to free-floating or sessile organisms. Many aquatic species display radial symmetry. Examples include echinoderms (starfish, sea urchins, sea cucumbers) and cnidarians (corals, anemones, jellyfish). Animals that can't move or are subject to currents must be able to find food and defend against threats from any direction. Most introductory textbooks list these animals as acephalic or lacking cephalization. While it's true none of these creatures has a brain or central nervous system, their neural tissue is organized to allow rapid muscular excitation and sensory processing. Modern invertebrate zoologists have identified nerve nets in these creatures. Animals that lack cephalization are not less-evolved than those with brains. It's simply that they are adapted to a different type of habitat.

Sources

• Brusca, Richard C. (2016). Introduction to the Bilateria and the Phylum Xenacoelomorpha | Triploblasty and Bilateral Symmetry Provide New Avenues for Animal Radiation. Invertebrates. Sinauer Associates. pp. 345–372. ISBN 978-1605353753.
• Gans, C. & Northcutt, R. G. (1983). Neural crest and the origin of vertebrates: a new head. Science 220. pp. 268–273.
• Jandzik, D.; Garnett, A. T.; Square, T. A.; Cattell, M. V.; Yu, J. K.; Medeiros, D. M. (2015). "Evolution of the new vertebrate head by co-option of an ancient chordate skeletal tissue". Nature. 518: 534–537. doi:10.1038/nature14000
• Satterlie, Richard (2017). Cnidarian Neurobiology. The Oxford Handbook of Invertebrate Neurobiology, edited by John H. Byrne. doi:10.1093/oxfordhb/9780190456757.013.7
• Satterlie, Richard A. (2011). Do jellyfish have a central nervous system? Journal of Experimental Biology. 214: 1215-1223. doi:10.1242/jeb.043687