Structures like mollusc shells, radiolarian skeletons and to some extent coral are formed through processes of growth like those discussed in Chapter 8. Structures like spider webs, wasp nests, termite mounds, bird nests and beaver dams rely on behavior determined by animal brains. (Even spider webs end up looking quite different if psychoactive drugs are administered to the spider.) And much like human artifacts, many of these structures tend to be distinguished by their comparative geometrical simplicity. In a few cases—particularly with insects—somewhat complicated forms are seen, but it seems likely that these are actually produced by rather simple local rules like those in aggregation systems (see page 1011).