Cephalization: Getting a Head Animals with bilateral symmetry typically exhibit cephalization (sef uh lih ZAY shun), the concentration of sense organs and nerve cells at their anterior end. This anterior end is often different enough from the rest of the body that it is called a head. The most successful animal groups, including arthropods and vertebrates, exhibit pronounced cephalization.

Close examination of insect and vertebrate embryos shows that their heads are formed by the fusion and specialization of several body segments during development. As those segments fuse, their internal and external parts combine in ways that concentrate sense organs, such as eyes, in the head. Nerve cells that process information and “decide” what the animal should do also become concentrated in the head. Not surprisingly, animals with heads usually move in a “head-first” direction. This is so that the concentration of sense organs and nerve cells comes in contact with new parts of the environment first.

Limb Formation: Legs, Flippers, and Wings Segmented, bilaterally symmetrical animals typically have external appendages on both sides of the body. These appendages vary from simple groups of bristles in some worms, to jointed legs in spiders, wings in dragonflies, and a wide range of limbs, including bird wings, dolphin flippers, and monkey arms. These very different kinds of appendages have evolved several times, and have been lost several times, in various animal groups.

In Your Notebook Explain in your own words why animals with heads tend to move in a “head-first” direction.

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