Amazon.com description: Product Description: Sponges are an ancient group of invertebrate animals whose basic body plan has remained unchanged for millions of years. There are no distinct organs or tissues, digestion is intracellular with excretion and respiration being by diffusion. The classification of sponges presents many problems because they share very few characteristics with other animals. Some zoologists consider them to be outside the line of evolution which led to other animals, the metazoans. Consequently, sponges may be labeled as parazoans (Gr. ????- beside; ???? - animal). This essential reference provides a complete list of all the known species of sponges (up to the end of 2007), along with a key to their synonyms. At the front of the book there is an outline classification to guide you to the location of unfamiliar species, followed by a synopsis of the phylum. This is followed by the checklist and synonym key. At the end of the book is an outline classification of the animal kingdom, and a brief bibliography.