Cephalization is the development of a head on an organism's body. Organisms like Porifera or Cnidaria lack a head region. Cephalization is usually found in organisms that are bilaterally symmetrical. These include: the Vertebrates--fish, amphibians, reptiles, birds and mammals. A head region is found in Invertebrates like the arthropods including insects, crustacea and arachnids, in Annelid worms and in Platyhelminthes. Having a head means that usually sense organs are located there and a brain can be found inside the head. This helps to improve an organism's response to stimuli and helps to coordinate and control the body processes.