The publication of this book was undertaken with two purposes in view: to bring together informatian on the deposition by living organ isms of unique skeletal structures composed of amorphous silica, and to review recent data on the involvement of silicon in physiological and biochemical processes. Although widely varying viewpoints are represented, all the contributors are very interested in the events in volved in the formatian of siliceaus structures and their function. Data presented deal with these questions in a variety of plant and animal systems, and at levels ranging from the evolutionary to the biochemical and ultrastructural. Innovations in electron microscopy and, indeed, the advent of electron microscopy itself, have stimulated many ultra structural studies of silica deposition, work which has deepened and widened the interest in those organisms which routinely produce "glassy skeletons. " The question of how silicon participates in biological systems in volves a spectrum of fields that indudes the chemistry of silicon per se, its biogeochemistry, biochemistry, ecology, and so forth. In this book, however, attention is focused up on the biological aspects of silicon and siliceous structures, with emphasis on the evolutian, phylogeny, morphology, and distribution of siliceaus structures, on the cellular as peets of silica deposition, and on the physiological and biochemical roles of silicon. This volume represents the first compilatian of such data. Because such a variety of subjects and fields are covered, the reader will have to glean for himself some of the comparative aspects of the data."

Modem biology owes much to the study of favorable model systems which fa cilitates the realization of critical experiments and results in the introduction of new concepts. Examples of such systems are numerous and studies of them are regularly recognized by the scientific community. The 1983 Nobel Prize in Med icine and Physiology is a magnificent example in which com plants served as the experimental model. In a manner somewhat more modest, other biological systems have attracted recognition due to their critical phylogenetic position, or indeed because of their uniqueness which distinguishes them from all other organisms. Assuredly, among the whole assemblage ofliving organisms, sponges stand out as worthy of interest by scientists: they are simultaneously models, an important group in evolution, and animals unlike others. As early as the beginning of this century, sponges appeared as exceptional models for the study of phenomena of cell recognition. Innumerable works have been dedicated to understanding the mechanisms which assure the reaggregation of dissociated cells and the reconstitution of a functional individual. Today, re search on these phenomena is at the ultimate, molecular level. Through an as semblage of characteristics the sponges are, based upon all available evidence, the most primitive Metazoans. Their tissues-perhaps one can say their cell groups-are loosely assembled (they possess no tight or gap junctions), cell dif ferentiation appears highly labile, and they do not develop any true organs. But, they are most certainly Metazoans."