ABSTRACT
One of the most striking features of vertebrates is their elaborate head, complete with a variety of sense organs, a large brain, and a skull. Vertebrates are, therefore, also known as craniates – “skull animals”. At first glance, having a head may not seem like a very remarkable distinction. Almost all bilaterally symmetrically animals – animals of the group Bilateria – have a front end with a mouth and a back end with the anus. Sensory organs tend to be concentrated at the front end, because this is the direction, into which the animal moves. Muscles and appendages associated with feeding or other environmental interactions are also concentrated there. The higher density of sensory and motor organs at the front end also requires larger neural control centers, such as enlarged cranial ganglia (concentration of nerve cells) or a brain. There may be special structures serving to protect the sensory organs and cranial ganglia from environmental impacts as well. We honor these specializations of the front end by calling it a “head”. However, not all bilaterian heads are equally complex. Truly sophisticated heads with an assortment of complex specialized sense organs evolved only a few times, most importantly in the arthropods and vertebrates.
