What do Bilaterian 'Head Genes' Tell us about the Evolutionary Origin of the Bilaterian Body Plan? - Marty Shankland & Ashley Bruce - from the abstract:
"The orthodenticle orthologue [gene] Lox22-Otx was isolated from the leech Helobdella triserialis...Patterns of head expression...mark tissue domains that exhibit discrete behaviors during later morphogenesis and differentiation....The largely head-specific expression of Lox22-Otx in this annelid supports data from two other bilaterian phyla (arthropods and chordates) in suggesting the existence of a genetically defined head/trunk distinction. We suggest that this head/trunk distinction is a synapomorphy of the Bilateria as a whole, and that it reflects some sort of regional or temporal distinction within the body plan of an early bilaterian ancestor....The bilaterian body plan is derived from a radially organized prebilaterian ancestor by the addition of a discrete 'trunk domain' that was situated anisotropically with respect to the axis of radial symmetry. The model suggests that the spatial restriction of 'head gene' expression in most Bilateria reflects a failure of these genes to be coopted into the early patterning of the trunk domain. Our model also portrays the anteroposterior [front - rear] axis of Bilateria as being novelty associated with trunk elongation. If this is the case, then AP organization would have been secondarily imposed onto the bilaterian head domain by its functional integration with the elongating trunk domain."
The following passage is from Hermann Popplebaum's, A New Zoology, pp.31-32. It is as far from the scientific biological approach given above as is possible to get. Yet both are studies in phenomena and nature - complementary and not (as the more extreme exponents of each position would claim) contradictory. Using the Anthroposophical paradigm, Poppelbaum explains various invertebrate phyla as excessive development of the human head (more correctly the archetype or morphotype of the head and skull). Organisms like Molluscs and Echinoderms are defined as "head-animals."
"We may begin by studying the characteristic "formative gesture" which coagulated into the human head. All soft parts of it are drawn together in the interior and encased in the hard skull as in a shell. Man carries the head in a resting position. The head can play its role the better the less it moves. Observing and musing, man faces the outside world with the help of his head. The sense organs do not stand out as protrusions. If the nose does, it gives a comical effect, "as if it wanted to raise claims which it is unable to substantiate" (Goethe). By comparing this picture with animal heads, the retention of prominences becomes all the clearer. We need only think of a pecking bird, or of the nodding head of a horse with its movable ears, or of the long antennae of a crab with its eyes on stalks. The human head, morphologically speaking, tends toward secrecy and repose. It withdraws into its protective casing.
Searching among animal types for similar tendencies, one is guided by such lower animals as are provided with shells beyond which the organs scarcely protrude or into which they can quickly withdraw. The best example is the shellfish (oyster or clam). Here, all soft parts huddle together and are wrapped in the mantle which builds the calcareous shell. There is no head as a separate organ. This animal is "all head". The most delicate parts, heart and kidney, occupy the safest innermost portion. Only the "foot" can protrude, helping by forward thrusts followed by contraction, to move the animal slowly on the sandy or muddy bed of the water. This foot is muscular and may be compared to a tongue, so the mussel is a head which moves laboriously by licking and sucking.
Starting from the shellfish, the other molluscs can be understood as a result of re-moulding. In the cuttle-fish (or squid) [Coleoidea] all sense bearing parts are conspicuously projected, especially those around the head, which appears here as the dominating organ with its huge eyes and tentacles. The shell fades into an inconspicuous soft scale (cuttle bone) which hides in the back of the mantle. (The fossil Ammonites still had a coiled shell.)
Here we grasp two contrasting modifications of the same type, the one retracting and wrapped up and the other rushing forth with greed. But if both variants of the mollusc type correspond to the human head, then a similar contrast must exist between two ways of using the head. Indeed, the quiet and reticent self-enclosure of the musing head, and the boundless greed of the sensuous face are both inherent as propensities in man.
There is also a balanced intermediary form between these opposites, the snail. Here the bulk of the viscera is hidden in a shell (and coiled up) but a head is clearly separated from it. The senses are, however, not thrust out, the eyes fairly simple, the feelers short stalks, and the foot a simple creeping sole. Thus we can arrange shellfish, snail, and cuttlefish in a chain of metamorphoses, each of which illustrates the head-nature of man without ever physically resembling the human head.
Another possibility inherent in the head is demonstrated in the Echinoderms (starfish, sea-urchin, sea-cucumber, and sea-lily). Again the inner organs are hidden in a hard shell, but their arrangement follows a strictly radial pattern of symmetry (mostly in five planes). The main systems, the water-vascular, the nerve- and blood-system, and also the gonads, are five-fold and arranged around the intestinal axis like the parts of a blossom around its center (see Chapter 19 for details). The whole architecture appears as an intellectual construction cleverly thought out and pedantically carried through. Physiognomically speaking, the Echinoderm brings a special propensity of the human head before our view, namely the careful and sure calculation of technical thinking which arranges tools and assembles them in a purposeful order, but without a margin for free imagination.
The basic forms of Echinoderms (star, sphere, disc, and cup) are of utmost simplicity in general, and yet intricate through the added details of divided arms (as in the gorgo-star), or in the many plantlike appendages of the sea-lilies which make them look like petrified flowers. The regularity of these structures, with the covering armor-plate units, reminds us of industrial mechanical production."
Hermann Popplebaum, A New Zoology, 1961, Philosophic-Anthroposphic Press, Dornach/Switzerland
The Head-Nature of Archosaur Evolution
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