AN understanding of physiological mechanisms in the nervous control of muscle must be preceded by knowledge of the anatomy and fine structure of the myoneural apparatus. Only rarely, as in the elucidation of nervous tracts, can the physiologist directly assist the anatomist. The anatomy must always be a little better known than the physiology. But at the present time there are enormous gaps in knowledge in both disciplines. This is particularly the case in regard to all invertebrate phyla, where there has been little interest in the fine morphology since the great era of comparative anatomy in the nineteenth century. Even then the accounts were usually no more than incidental parts of general descriptions. Also the functional approach, which would have led investigators to ask what, from the present viewpoint, are the right questions, had not at that time been developed. The newer interest in the field, which is now developing rapidly, is the result not of fresh anatomical researches, but of the inquiries of experimental investigators. These workers, finding themselves armed with powerful resolving techniques which might be expected to solve many functional problems, can only hope that a new generation of anatomists and histologists will arise to establish the morphological data on which basis alone they may proceed to an understanding of function.
The muscular apparatus of many metazoan invertebrates, instead of being formed of discrete anatomical muscles, frequently consists only of layers of scattered muscle cells embedded in, or attached to, sheets of connective tissue. Muscle cells are encountered even in those Metazoa (the sponges) which possess the lowest level of structural organization. Here they occur in isolated groups where their contraction serves to perform the simple functions of retraction and constriction of oscula, but sponges have no nerve cells, so excitation of these muscle cells must be due entirely to local stresses.