Biological systems can be modeled beneficially as reactive systems, using languages and tools developed for the construction of man-made systems. Our long-term aim is to model a full multicellular animal as a reactive system ---- specifically, the C. elegans nematode worm, which is complex, but very well-defined in terms of anatomy and genetics. The challenge is to construct a full, true-to-all-known-facts, 4-dimensional, fully animated model of the development and behavior of this worm (or of a comparable multi-cellular animal), which is easily extendable as new biological facts are discovered.