Cephalisation - Metazoa
Bilateral animals when creeping or swimming, have a tendency to keep the same end of the body forward and the same surface down towards the substratum. In such a case the sensory organs and nervous system would also have a tendency to be concentrated at the anterior end. This differentiation of a 'head end' is known as cephalisation'(literal1y head development). Cephalisation has evolved to various degrees in bilateral animals.
Figure: Concentration of nervous system in anterior region of a) liver fluke b) Tapeworm c) Planaria
The mouth is usually located at the leading end with which become associated the organs for food capture, as the sensory organs on the head can detect food. Neurons become organized into brain in this region for rapid coordination; longitudinal nerve cords are developed for rapid transmission of information throughout the length of the body. Cephalisation in its most primitive form can be seen in Platyhelminthes.