In this paper we attack the problem of understanding the localization of the main structures involved in the motor circadian rhythm of crayfish by analysing its ontogeny. We present experimental results giving the properties of this rhythm in young and adult crayfish. Then we construct a mathematical model (based on a previous one for the electroretinogram circadian rhythm in the same species) simulating those properties. In the process of constructing the model we clarified and made precise various hypotheses about the biological structures involved in them and about the characteristics of the oscillators present in those structures. We also formulate some hypotheses about the general properties of circadian rhythms. Finally, we propose some experiments suggested by the mathematical model.