An enormous amount of new information on in vertebrate animals appears in the research literature each year, and with some justification each edition of Invertebrate Zoology has grown in size. This edition has therefore been reduced by approximately 170 pages. The re- duction was made by removing some detail from the text and eliminating redundant and less useful illustrations. However, the contraction was not made at the expense of new material, which was added before the final trimming, nor at the expense of the level of depth that has characterized this textbook since its inception.
    
A new feature of this edition is short, boxed essays on topics such as-bioluminescence, invertebrate eyes, and trochophore larvae that are applicable to more than one group of invertebrates. These essays appear in the first place in the text in which they are relevant, and they are referred to elsewhere in the text. 

Various invertebrate groups have been moved about in this edition to reflect new ideas on phylogeny. Thus, annelids now precede the molluscs, onychophorans have been returned to the chapter on lesser protostomes, and priapulids have re-joined the aschelminths. I have always attempted to use a system of classification that is up to date but generally accepted. In some cases, I have let pedagogy be the determining factor. For example, it is Simpler to discuss the cubomedusae with the scyphozoans than as a separate class, and protobranchs, lamellibranchs, and septibranchs are still very useful reference groups for bivalves. Nevertheless, the student is made aware of current thinking on the classification of these animals. 

I continue to believe that this text should provide the student entry to the vast literature on in- vertebrates. The references at the end of each chapter therefore include most books and review articles on invertebrates, as well as many research papers. Some of these would be appropriate for further student reading if the instructor wished to so designate them. The listings for the larger chapters have been subdivided to make them easier to use. The glossary, which continues to be restricted to recurring terms, has been expanded and combined with the index.