This study focuses on the concept of a 'model organism' in the biomedical sciences through an historical and philosophical examination of research with the nematode Caenorhabditis elegans. I explore the choice of C. elegans in the mid-1960s, showing a rich context existed within which the organism was selected as the focus for a molecular biological research program, including an experimental life prior to Sydney Brenner's work. I argue that this choice can be seen as an obvious outcome of what was (...) a complex search procedure, and that the success of the 'worm project' depended not only on organismal choice but also on the conceptual and institutional framework at the Laboratory of Molecular Biology in Cambridge within which it was pursued. Examination of C. elegans work in the late 1960s through the early 1980s illuminates several additional theses. Although development and behavior were the general areas of interest for the project, the original goals and proposed methodology were extremely vague. As the project evolved, which investigations proved to be tractable using the worm depended not only on which methodologies were fruitful but also on the interests and skills of early workers. Much of the power of C. elegans as a model organism can be traced historically to the investment of resources which resulted in an unprecedented, complete description of the organism, and represented a return to a more naturalistic biological tradition. In light of the historical study, I develop a philosophical analysis of C. elegans as a model organism through exploration of the three kinds of modeling with the worm: modeling of structures, processes, and information. I expand the notion of a descriptive model, arguing that C. elegans is best understood as a prototype of the metazoa, and that C. elegans as a model organism has been not only heuristically valuable, but also essential to this research project particularly in its pre-explanatory stages. I conclude by suggesting that more investigation of descriptive prototypes such as those in the worm project must be done to capture important aspects of the biomedical sciences that will be neglected if explanatory models are the sole focus in the philosophy of science. (shrink)