I have built this book from two of my previous ones, The Invertebrates and The Chordates. Together, they surveyed the animal kingdom (including the protozoans) in 1072 pages. This book covers the same ground in much less space. I believe that this shorter treatment will be found more suitable for modern university courses, which devote less time than older courses did to the structure and diversity of animals. I expect the book will be used mainly by undergraduates in their first or second year.

This book is about the major groups of animals, about their structure, physiology and ways of life. Each chapter, except the first, deals with a taxonomic group of animals, usually a phylum or class. Brief descriptions of a few examples are followed by more detailed discussion of selected topics. Some of these topics are peculiarities of the groups (for instance, the shells of molluscs and the flight of insects). Others are more widespread features or properties of animals which can be illustrated particularly well by reference to the group. Thus I have used jellyfish in chapter 3 to illustrate the workings of simple nervous systems, and have explained some of the basics of muscle physiology in my account of molluscs, in chapter 6. I have described many experiments because I think it as important and interesting to know how zoological information is obtained, as to know the information itself.

The diversity of animals is a major problem in writing about them. Any attempt at encyclopaedic coverage results in an enormous quantity of indigestible morphological and taxonomic information.