Techniques developed for enclosing viable natural planktonic ecosystems pro vided the opportunity for prolonged and detailed investigation of dynamic events within the pelagic system of a known water body. Recent investigations into plankton ecology, using enclosure systems in dif ferent marine environments, are discussed in relation to the data obtained from the Nanaimo, British Columbia, Canada, plastic-sphere experiments of 1960 and 1962. Three types of modern enclosure experiments are recognized: floating systems within nutrient levels maintained or running down, and benthic attached systems. The review largely discusses results from the two kinds of floating systems. Processes at several trophic levels have been investigated in enclosures. This review attempts to draw together details from all experimental systems to emphasize the enclosures' contribution to our understanding of planktonic systems. Enclosures made it possible to examine primary production processes, particularly in relation to inorganic nutrient availability and water-column sta bility. Recent experiments have used the understanding of these processes as a management technique in maintaining different planktonic systems. Relation ships between primary and secondary trophic levels are not always easy to inter pret, since the growth of primary carnivore populations can often determine the survival of zooplankton populations. Nevertheless, the development of co horts of herbivorous zooplankton has been followed in several enclosures, yield ing useful information on development times and production rates. In enclosed systems it is thus possible to directly relate tertiary level production to inorganic nutrient input, and to calculate production rates and exchange efficiencies at several trophic levels.

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.

The Biology of Tardigrades is the only modern comprehensive account of marine and freshwater tardigrades. It contains the first taxonomic review for over a decade in addition to covering every aspect of their biology.