Cephalization describes the process by which organisms develop a distinct head. The head of a cephalized organism contains a concentrated group of nerves, or brain, that controls the rest of the organism, as well as specialized organs for consumption and perception, like mouths, eyes and ears. Cephalized organisms exhibit a distinct division between parts of the body; they have a front, back, top and bottom. These animals comprise the bulk of animals that are encountered on a daily basis.

All vertebrate animals qualify as highly cephalized organisms on account of their distinct heads, well-developed brains, elaborate nervous systems and complex thought processes. Examples of highly cephalized vertebrates include humans and other primates, such as gorillas, chimpanzees, baboons and bonobos; domestic animals like cats, dogs, ferrets and rabbits; common pest animals like rats, mice, squirrels and raccoons; and large mammals like bears, deer, lions, elephants, pigs, horses and sheep. Other vertebrate animals include:

• lizards
  
• snakes
  
• amphibians
  
• birds
  
• bats
  
• fish

Any organism with a backbone is a vertebrate and exhibits a high degree of cephalization.

Cephalopods constitute a group of invertebrate animals that nonetheless exhibit a high degree of cephalization. These animals comprise a group of mollusks with centralized heads and highly developed brains. The four types of cephalopods are octopuses, squid, cuttlefish and nautiluses. In addition to having distinct heads and highly developed brains, these animals possess the ability to change color, texture and body shape to elude predators.

Insects exhibit cephalization because they have unique heads with special parts for consumption and perception, and the distinct division of body parts. They do not, however, exhibit a high degree of cephalization, as insects lack the complex thought of vertebrates and cephalopods. Some insects exhibit cephalization of the head, but not of the nervous system. Arachnids, a group including spiders, mites and scorpions, among other organisms, also exhibit cephalization. Flatworms of the Platyhelminthes phylum show the beginnings of cephalization but are not fully cephalized.

So many organisms exhibit cephalization that describing noncephalized animals may be the easiest way to give an idea of just how many animals are cephalized. A handful of ocean animals possess basic nervous systems devised as a nerve net, with no head. These organisms include Cnidarians, such as coral, jellyfish, sea anemones and simple mollusks like scallops. Echinoderms, or sea stars, also lack cephalization. Nearly all animals not falling into one of these categories exhibits some degree of cephalization.