We are all familiar with many of the organ systems that comprise the body of advanced animals: such as the circulatory system, nervous system, etc. More of us are aware of the essential nature of the immune system in these days of HIV, AIDS, and emergent viral diseases such as Ebola and Hanta. Later chapters will focus on animals, such as sponges that have no organs at all, and other organisms that lack many of the organ systems we take for granted.
