HAVING collected together information regarding the nervous control of the muscles of both the internal organs and the skeletal apparatus of diverse groups of animals it is perhaps disappointing to find no neat evolutionary story coming to light. Important differences between phyla certainly have been exposed, but one is left with the impression that evolution has been surprisingly content with its earlier efforts in myoneural architecture and function. The evolutionary possibilities which opened up as the result of such innovations as bilateral symmetry, an articulated skeleton, air-breathing or warm-bloodedness, are not paralleled by any correspondingly spectacular changes in myoneural phenomena.
A striking feature of the advanced groups is the marked striation of the somatic muscle fibres; but striation as such is also observable in some fibres of primitive animals, even in the coelenterates. Striation seems to appear in an invertebrate animal whenever a high speed of contraction is required in a functionally important muscle. Although striation generally appears to be associated with speed of contraction there is the anomaly that all the insect fibres, including those of the gut, are striated. In the vertebrates the striated fibres are the only ones which are under voluntary control. Their presence in the anterior part of the gut can therefore be associated with the voluntary act of swallowing. The meaning of the presence of striated fibres throughout the gut in some fishes, however, is obscure, as is also the presence of a ring of pure striated, but involuntary, muscle, as an anal sphincter in mammals.
There is ample evidence for the presence of several motor nerve terminations on individual muscle fibres (multiterminal innervation) in many invertebrates from various phyla, and it may be that this is quite widespread, or even universal, in