JTW's Evolutionary Origins - Author: Romer, Alfred Sherwood
HOME
The Vertebrate As A Dual Organsim: The Somato-visceral Animal
"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 it's appendages, which, to a considerable degree, conducts it's own affairs, and over which the somatic animal exerts but incomplete control."
(Romer, 1972)
"A consideration of what evidence can be gathered from our knowledge of living vertebrate relatives among chordates or hemichordates suggests that this dichotomy has a historical background; that the remote chordate ancestor, as exemplified by pterobranchs, acorn worms and tunicates, was essentially a small, sessile, simply built animal that included in its structure little but a food-gathering and digestive apparatus; that in the developmental stage of more advanced chordates there was added to this, for better distribution of the young, a locomotor "unit" with muscles, supporting structures, and brain and sense organs; that this newly acquired "somatic animal" was at first restricted to the larval period, and resorbed for adult existence; but that, eventually, in progessive chordates, the new, somatic unit was retained throughout life. The whole story of vertebrate evolution from protochordate ancestors has been one of the elaboration of this somatic unit, ..."
(Romer, 1972])
"It is not unreasonable to speculate on the possibility that the ancestor of 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 postganglionic neurons may be representatives of the original visceral nerve net system, the preganglionics representatives of attempts at domination by the central nervous system."
(Romer, 1972)
"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."
(Romer, 1972)
- Romer, Alfred Sherwood
- The Vertebrate as a Dual Organism: The Somato-Visceral Animal
- Evolutionary Biology: Vol. 6, pp. 121-156
- 1972
HOME