The Somatovisceral Animal - The Vertebrate as a Dual Animal
The Vertebrate as a Dual Animal: Somatic and Visceral
"In many regards the vertebrate organism, whether fish or mammal, is a well-knit unit structure. But in other respects there seems to be a somewhat imperfect welding, functionally and structurally, of two somewhat distinct beings: (1) an external, "somatic," animal, including most of the flesh and bone of our body, with a well organized nervous system and sense organs, in charge, so to speak, of "external affairs," and (2) an internal, "visceral," animal, basically consisting of the digestive tract and its appendages, which, to a considerable degree, conducts its own affairs, and over which the somatic animal exerts but incomplete control."[2]
The Functional Welding of the CNS to the ENS - The Autonomic Nervous System
"To sum up the phylogenetic suggestions gained from a consideration of the structure of the nervous system in living vertebrates, high and low, and of their chordate and protochordate "ancestors," one tends strongly to gain the impression that the remote "visceral" ancestral form had a simple superficial nerve net and, at some early stage, acquired a visceral nerve net as well; that, with the development of the "somatic" animal, there developed the central nervous system, with segmental nerves including a ventral root of somatic motor type and a distinct dorsal root at first composed merely of somatic sensory neurons; but that there was a strong tendency for the somatic animal to attempt neural control over the visceral animal, first perhaps, by a direct connection with the important visceral muscles of the pharynx, later by an attempt to dominate the gut by autonomic fibers, originally by way of dorsal nerve roots, running to the postganglionic neurons, which represent elements of the original gut nerve net. The development of visceral centers in brain and cord was associated with this attempt at domination of the visceral by the somatic animal. But, as we are ourselves aware, the integration of the visceral animal into the dominant nervous system of our somatic being is still far from perfect."[2]
The points of fusion between larval and adult bodyplans - Unmyelinated parasympathetic system.
Tunicate adult bodyplan - Pharyngeal arch system with visceral enteric nervous system (ENS).
Tunicate larval bodyplan - Somatic CNS, head senses, notochord, segmented musculature, and post anal tail.
Gaining control - Myelinated sympathetic system exerting control by the somatic CNS over the visceral ENS.
The Functional Welding of the CNS to the ENS - Gaining Control
"It is not unreasonable to speculate on the possibility that: The ancestor of the vertebrates may have had, like many invertebrate types, an essentially independent visceral nerve net; that the development of the autonomic system represents an attempt by the central nervous system at gaining control over visceral activity; and that possibly in the peculiar two-neuron system seen here, the post-ganglionic neurons may be representatives of the original visceral nerve net system, the pre-ganglionics representatives of attempts at domination by the central nervous system."[2]
Evolution of the Pharyngeal Arch Cartilages and the Striated Visceral Musculoskeletal System
"In every vertebrate there is a set of well-developed striated muscles associated with the anterior part of the digestive tract, most notably the pharynx. In fishes there is an important series of muscles, lying along the walls of the pharyngeal region, which effect opening and closing of the gill slits. In jaw-possessing fishes there are powerful muscles associated with jaw movements; it is universally agreed that the jaws represent a modified and enlarged series of gill bars, and these jaw muscles are clearly a special part of the pharyngeal series.In tetrapods the jaw muscles remain prominent, but the gills are lost; much of the original gill musculature disappears, but a few small muscles persist in the throat and ear region, the trapezius muscle system of the neck is a further persistent relic, and in mammals the muscles of expression are an outgrowth of the same set of muscles."[2]