No characteristic of animal functioning is more distinctive than the power of movement. But the possession of tissues having the capacity for doing mechanical work is of no consequence unless a means exists for harnessing the latent power and releasing it, in a manner calculated to serve the functional needs of the animal. The achievement of this control is the primary function of the nervous system. The control process has so many aspects that several divisions of the science of physiology are included. There are the speed and power of the whole muscle in relation to load and excitation, under different environmental conditions, which set limits to the extent to which complete control is possible; there are the functioning of the motor nerves, the mechanism of transfer of excitation from nerve to muscle, and all those reflex aspects of the functioning of the central nervous system such as were studied by the Sherrington school. We may also add the study of the muscle proprioceptors and the associated servo-mechanisms. Indeed, the whole of the nervous system (including the autonomic nervous system) is associated directly or indirectly with the control of muscle.
There is no reason to doubt that motility and irritability are two distinct physiological processes in every instance in which they are encountered. But the two are coupled together in intimate fashion. It is probable that a coupling mechanism relating muscle-cell irritability to contraction existed before nerve cells were evolved. A similar link is probably present in an elementary state in the muscle cells of the sponges.
The simplest way in which the evolving motor nerve cells could have influenced motility was to affect the irritability of the muscle cells, a direct influence on the contractile elements in the cells probably being much more difficult to attain. Was this the case? If so, what is its mechanism? Do all animals have similar mechanisms? If not, what is the variety of mechanisms encountered and how has it evolved? These are some of the problems which it is the task of the comparative physiologist to explore and with which the present monograph is concerned. Since a myoneural apparatus is present in all phyla except the