Without its robust calcified exoskeleton, a crab would be very vulnerable prey. Its pincers would be harmless; its whole locomotory apparatus would be at loss. The same applies to a mammal, or a bird, without its internal skeleton. If the adult is to be endowed with a complete, functional skeleton, the business of constructing one must start early in development. But how and why were skeletons invented?

If an explanation of what happens in development is to be searched for in development itself, as argued in chapter 1, then the reasons for the first appearance of a skeleton must be sought for in development rather than in mechanics. I am not speaking of the heavy armour of an adult crab nor the sophisticated architecture of a vertebra. I am speaking, instead, of the reasons why some early multicellular organisms found it profitable to produce a cuticle, or to adventure into the previously unexplored paths of biomineralisation. To place this question in a plausible context, we need to digress some.