The evolution of animal diversity is strongly affected by the origin of novel cell and tissue types and their interactions with each other. Understanding the evolution of cell types will shed light on the evolution of novel structures, and in turn highlight how animals diversified. Several cell types may also have been lost as animals simplified - for example did sponges have nerves and lose them? This book reveals the interplay between gains and losses and provides readers with a better grasp of the evolutionary history of cell types. In addition, the book illustrates how new cell types allow a better understanding permitting the discrimination between convergence and homology.