Ea: Pentameran origins and homologies

« I went for a stroll through the market road of Babatunde, and there were rows of sun-dried trepangfish hanging from the stands, deepfried palps handed out in paper bags with rice and beans. Others were dried and turned into straws. How strange, for an animal-like creature to have multiple mouths and guts in appendages at one end of the body: why not in the middle, like in sea cucumbers or jellyfish? The answer struck me. Organisms are not drawn from scratch, but modified versions of their ancestors. Wasn't there, in our own seas, a simpler creature with a mouth at the end of each limb? What were the limbs of land 'vertebrates', with their ocelli and digestive glands, but modified heads? Eoastrians – the humble mudstars – were the wellspring of all the greatest beasts of Ea. » – Cornelius Adebayo, World in a Raindrop: An Autobiography, Nisaba Press, 112 AL

The ur-Pentameran was likely something similar to a modern fingerstar, with two overlapping sets of five arms. Each of the upper or principal arms ended in a proto-head comprising of a boring proboscis protected by a fingernail-like shield and a simple eye. The nervous system is diffused, with a ganglion associated to each eye and connected to all others by nerve fibers. The digestive system is similarly decentralized with the opening of each arm serving equally as mouth and anus, and connected to a central annual (ring-shaped) stomach. This arrangement probably originated from a five-fold clonal colony of a worm-like organism with a blind-end gut. It's not yet clear whether the shields, eyes, and probosces appeared in the worm-like progenitor or only later, in the colony.
This structure is preserved in modern Eoastria, and was subsequently modified in three different ways. In Actinognatha, the body elongated along the apico-basal axis, with the principal arms clustering at one end, and the accessory arms at the other. In Simplostomia and Diplostomia (the "geopentamerans"), respectively one and two principal arms form a head at the anterior end of the body, while the others become specialized as locomotory limbs.

1. Principal/apical arms: five, radially symmetrical, in Eoastria, to collect food and brood egg masses. In Actinognatha they converge on the apical end of the body and become the buccal palps; a vestibule may form at their center to store food before it's transferred to the mouths. In Simplostomia, one arm forms the head, while the other four become two pairs of limbs; in Diplostomia, two arms partially fuse to form the palps or pseudojaws, leaving three to become a pair of forelimbs and an unpaired hindlimb.
2. Accessory/basal arms: located just below the principal arms in Eoastria; function unknown in most species, though filter-feeding ones use them to adhere to the substrate in strong water currents. In Actinognatha they become the aboral cerci, which propel the body when swimming, lock partners during fertilization, and sometimes carry egg masses. They disappear early in embryonal development in both geopentameran clades, though they may be involved in the formation of ventral plates and muscles, and possibly the labium.
3. Proboscis: this muscular retractile organ collects food in a variety of ways in Eoastria. In Actinognatha, all proboscides form the palp teeth. In Simplostomia, the proboscis of the head generally forms a calcified stylet, many species being liquivores. In Diplostomia, the probosces of the pseudojaws form a radula (grinding tongue). In both geopentameran clades, the limb probosces form part of the claws and of the limb-tip musculature; in many Diplostomia, the hindlimb claw is developed as defensive weapon or to carry eggs. The sensitive coating of the proboscis forms the olfactory flaps of Simplostomia, as well as the ear, tongue, and labium of Diplostomia.
4. Salivary glands: one connected to each proboscis, produce digestive enzymes in all Pentamera. The enzymes of liquivorous Simplostomia are particularly powerful, resembling Terran spiders in this regard. In many geopentamerans of both clades, the salivary gland of limbs becomes a venom-producing gland, connected to the claw.
5. Ocelli: an ocellus or primitive eye, formed by a clear calcite lens covering photosensitive tissue, is located on each principal arm of all Eoastria, just above the ungueal shield. In Actinognatha they are all equally developed, whereas in Simplostomia the one located on the head developes into a fully image-forming eye, and so do the two on Diplostomia pseudojaws. Both geopentameran clades can have ocelli on their limbs, in various stages of development (including being fully vestigial); the one on the hindlimb of Diplostomia is especially important, and in some herbivorous species it directly controls venom production in the nearby gland.
6. Annular stomach: in Eoastria, each arm-tip hosts a mouth, and all are connected to a ring-shaped stomach in the body, with an internal ciliate coating that produces an internal current (counter-clockwise when seen from above); waste is expelled through whatever mouth is not currently eating. In all geopentamera, food is ingested through a single mouth. In Simplostomia, this mouth is located under the head, often inside the stylet, and waste is expelled through mouth-derived pores in the hindlimbs. In Diplostomia, the mouths of the pseudojaws merge into one opening, which serves both to ingest food and to expel waste. The latter also occurs in Actinognatha, in which often the ring extends caeca into the trunk.
7. Ungueal shield: a particularly large shield plate that protects each arm-tip in Eoastria. In Actinognatha it forms part of the teeth in all palps. In Simplostomia it grows into the cephalic shield which envelops the whole head except for eye and mouth, and reinforces the stylet; the shield is very important in fossil identification, as often it's the only non-fragmentary remain. In Diplostomia the shields of the two pseudojaws migrate ventrally, articulate at the base, and form the maxillae. In both geopentameran clades, the shields on limbs become the coating of claws.
8. Genito-respiratory sacs: they open with a slit or pore between the principal arms in all Eoastria, in which they often retain eggs or even larvae. In Actinognatha the respiratory and reproductive functions separate, producing genital slits and branching "water lungs". In geopentamerans the sacs specialize differently. In Simplostomia, the sacs between head and forelimbs become the male genitalia, and the one between the hindlimbs the female genitalia, so that mating occurs "shoulder to tail"; the sacs between forelimbs and hindlimbs become lungs. In Diplostomia, both male and female genitalia develop from the sac between the pseudojaws; the remaining sacs disappear, and respiratory organs form from the dorsal trabecular system.
9. Nervous ring: all Eoastria have five nervous ganglia, each located near an ocellus, directly connected to a ring of nerve fibers in the central body. In Actinognatha, this arrangement survives, though the ganglia are reduced, and most cell bodies are concentrated in the ring, which is found in the collar just behind the palp roots. In Symplostomia, the ganglion of the head forms the primary brain, connected by cephalohumeral commissures to the humeral ganglia at the base of the forelimbs, and these are in turn connected by humeropelvic commissures to the pelvic ganglia at the base of the hindlimbs. The arrangement in Diplostomia is similar, though two ganglia fuse to form the brain, and there is only one posterior ganglion.
10. Hydromuscular system: in Eoastria and Actinognatha, the hydromuscular system is composed of a number (5-50) of interconnected spherical micro-reservoirs, or hydrocysts. The hydraulic fins of Actinognatha derive from hydrocystal outgrowths, probably with an initial respiratory function. In all geopentamerans 8-14 hydrocysts merge into the segmented Vindhana organ, whereas the others disappear early in embryonal development.

– from C. Hansen, I. Li (eds.), Fundamentals of Ean Developmental Anatomy, Leewenhoek Academic Press, 233 AL

The peculiar distribution of Geopentamerans on Ea's continents has long been a puzzle. In the present work, we bring together multiple lines of evidence to suggest that land-dwelling Pentamerans have evolved from the beginning on separate continents with limited interaction. Molecular evidence have firmly established that Geopentamera is not a monophyletic group, and that Simplostomia and Diplostomia arose independently from Eoastrian ancestors. This has led to Harbi's model of the "amphibious jungle" populated by hypothetical land-dwelling Eoastria, which gradually developed into Simplostomia and Diplostomia as the continents separated. Geological and paleomagnetic evidence show that Inanna, Dumuzi, and Enlil were united in a supercontinent no later than 200 million years ago (Ma), and are indeed still diverging. This supercontinent, tentatively named Punt, is the most obvious site for Harbi's "amphibious jungle"; but this raises the question of how could the macro-pentameran fauna sort itself so precisely on the following continents, and why did not one of the two clades outcompete the other at an early stage. On molecular and paleontological grounds, we propose that terrestrial Pentamerans arose no earlier than 150 Ma, and possibly as recently as 100 Ma, when the continents were already separated by wide tracts of sea. The origin of Simplostomia and Diplostomia represents therefore two roughly simultaneous, but separate and wholly independent, events of colonization on land, taking place the former on Dumuzi, the latter on Inanna. Later migrations, due to rafting events or the evolution of more motile species, occurred only when the two faunas were well entrenched. Thus, the fauna of Inanna is dominated by Diplostomia (except for flying species) and that of Dumuzi by Simplostomia, whereas both remain marginal on Enlil, where their presence is relatively new. We suggest that the present period of Ea's biological macrohistory is roughly analogous to the Carboniferous and Permian periods of Earth's, that is, corresponding to the first appearance of land-based macrofauna.