The history of Invertebrate Zoology over the past 40 years can be used to illustrate interest in organisms and some of the ways in which the symposium's question may be interpreted. The study of animal organisms from a holistic perspective has progressed enormously as reflected in changes in described and estimated numbers of species, in the discovery of new higher taxa and in the growth of literature. Generalizations on the biology of animal organisms, however, rest on relatively small samples, and many of the same organisms that have received the most attention in the past continue to receive the most today. Symbiosis and colonial organization have been two important means whereby new organizational levels for organisms have evolved. Ultrastructural research over the past 20 years has provided new evidence in support of the hypothesis promulgated long ago that multicellular animals (metazoans) may have evolved from colonial protistans. Some polymorphic, colonial metazoans have approached or crossed the threshold to a still more complex level of organism.