What is Cephalization?

Cephalization is a steady evolution process through which the mouth, sense organs, nervous, and sensory tissues become concentrated at the anterior part and make the head region. Cephalization word is derived from the Greek word Kephalē, which means head, so in simple words, cephalization means "having a head."

Cephalized Animals

Animals that undergo cephalization mostly show bilateral symmetry. All vertebrates, cephalopods, mollusks, annelids, and arthropods show a high degree of cephalization.

A flatworm is a primitive organism with a bilateral body consisting of sensory cells and the brain at the anterior end. Still, unlike other cephalized animals, it has a mouth at the center of the body. Flatworms' nervous system looks like a ladder, and they have a small ganglionic brain.

Arthropods and annelids have ganglionic brains at the anterior side connected to the ventral nerve cord. In arthropods and hexapods, three pairs of ganglions fuse to make the brain. Each pair of ganglions is helpful in the detection of a limited range of the spectrum. Axons act like bridges between the brain and wings that provide quick transmission of nerve impulses. The arthropods also have a pair of antennae that act like sense organs present at the anterior end. 

Some mollusks, like cephalopod mollusks, show cephalization. In mollusks, a complex nervous system is related to their habitat. Squids, octopuses, and snails are some examples of mollusks that have a complex nervous system. Cephalopod mollusks have large brains, big camera eyes, and big axons for quick conduction. They are good aquatic predators due to their vertebrates, like eyes with iris, circular lens, and photoreceptors that detect light. In fact, in all invertebrate cephalopods, mollusks have the most complex nervous system.

All vertebrates have a well-developed nervous system and can perform complex thought processes. Invertebrates, the primates, have a high degree of cephalization. Like other chordates, vertebrates have nerve cords that contain clusters of neurons that run throughout the body. The vertebrae nervous system is divided into the central nervous system (CNS) and peripheral nervous system (PNS). The central nervous system contains the brain, and the spinal cord protected by the backbone, and PNS contains all the nerve tissues. This type of division shows that the human nervous system is highly developed. In invertebrates, divisions of the brain are observed (forebrain, midbrain, and hindbrain). These divisions help perform different and complex tasks. Therefore, the vertebrates' brains are more complex than the invertebrates' brains.

Some animals that do not possess bilateral symmetry also show some degree of cephalization. For example, a hydra(cnidarian) has a radially symmetrical body but has a head that contains neural cells, photoreceptive cells, and the mouth. In hydra-like other cnidarian, neurons network is present throughout the body. Still, due to the presence of sense organs and photoreceptive cells closer to each other, they are said to have some degree of cephalization.

By observing hydra, flatworms, arthropods, vertebrates, and other animals, it is clear that cephalization also results in the evolution of neurons to ganglions to brains in animals.

Key Features of Cephalized Animals   

• Show bilateral symmetry.
• Presence of sensory organ near the anterior part.
• Mouth near the front of animals.
• Well-developed nervous system(presence of neurons, ganglionic brain, and brain).
• Able to perform quick and complex movements.
• Centralized brain (it has been observed that centralized brain is the key feature in bilaterians).
• The maximum concentration of neurons at one point.

Advantages and Disadvantages of Cephalization

It is helpful in the growth of the brain, the control center of all activities. The presence of all sensory organs at the front part helps the organism detect its environment efficiently. This organization also allows animals to easily find their food, shelter, mate, and sense danger. The front part of animals comes in contact first with their environment. Hence, the presence of the nervous system and sense organs at the anterior end is helpful to collect information about the surroundings more quickly. Cephalization results in a well-located and developed nervous system that can control and organize all sensory information. The other advantage is the presence of a mouth near the sense organ that helps an organism quickly detect its food sources, especially in predators. Most predators have special sensory organs near the mouth, like in the case of cats that have whiskers to detect prey in the dark.

There is one disadvantage of the cephalization process that it requires a lot of energy.

Non-Cephalized Animals

Unlike cnidarians, all bilaterian Porifera do not possess a nervous system, so Porifera does not show cephalization.

Cnidarians and echinoderms are some groups of animals that do not possess a head region at the anterior end. Unlike cephalized animals, their bodies do not contain more neurons in one part. In cnidarians and echinoderms, the nervous system is not concentrated at one end of their body; a network of neurons is spread throughout the body. 

According to scientists, primarily aquatic animals like cnidarians and echinoderms do not require cephalization. Their body can perform the function without undergoing the process of cephalization. Most animals that do not have to move for their food are classified as non-cephalized animals. In simple words, they do not need a complex nervous system to do their life processes without having a head region.

The Evolutionary Trend in Cephalization

Chordates, arthropods, and annelids are some examples of advanced animals that undergo cephalization several times. Scientists have observed that the animal that is more evolved and advanced has experienced cephalization and has more neurons in one place. In other words, it can be concluded that cephalization is linked with the complex nervous system of animals. But in some cases, this evolutionary trend is not valid such as in starfish. The starfish is not cephalized but is considered an advanced evolutionary organism.

Context and Applications

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for

• Bachelors in Biology
• Bachelors in Microbiology
• Bachelors in Neurology
• Masters in Microbiology
• Masters in Microbial Biotechnology
• Masters in Molecular Genetics

Related Concepts

1. Encephalization
2. Evolution of the brain
3. Centralization and symmetry 
4. Spatial perception and central nervous system symmetry
5. Diversity of nervous system
6. Neogenesis
7. Organogenesis
8. Phylogenetics