Ord: Polybrachs

Introduction | Spherindricites > 

As we saw in the introduction, Ord is a gigantic place. There is enough room on Ord for life to have arisen completely independently several times, and for hundreds of completely unrelated alien civilizations to develop--even though, if they knew which way to walk, they could find each other within a few thousand kilometers.

We will be looking at the development of only one branch of animal-like life. At the highest level, this branch of independently-evolved animal life in Ord's oceans and seas can be split into three groups: sponges, flatworms, and polybrachs. Ordian sponges are much like Earthling sponges--simple sessile colonies of cells which filter food particles from water flowing through them. Ordian sponges, however, are "more spongy"--more porous--than Earthling sponges can be. This is because the four-dimensional space they live in permits qualitatively larger holes, of a fundamentally different kind than exists on Earth. Ordian matter can have linear holes punched through them, just like we can, but they can also have planar holes--and Ordian sponges do, because it allows more water to flow through them from more directions.

Flatworms are spheroidal organisms; they would not look flat to us, but they are flat on Ord, as their entire lower 3D surface can contact the ocean floor simultaneously, and they have very little extent in the upwards direction. These organisms show minimal layered tissue differentiation. Simpler species are completely spherically symmetric, and simply absorb nutrients from stuff they crawl over as they inch their way across the ocean floor. Some more derived species, however, have established a front-back axis specialized for motion; such creatures have more elliptical bodies, and can often be found freely swimming in the ocean bulk.

The flatworms may eventually produce more interesting descendants, but for now the most complex creatures are the polybrachs. These are also spherically-symmetric creatures with an up-down axis, but they have specialized arm structures improving their ability to navigate and manipulate their world. Their symmetrically-arranged body segments and attached arms make them somewhat analogous to Earthling starfish, but with one major difference: while different species of starfish may have have any number of equally-spaced arms, due to the fact that there are infinitely many regular polygons in two dimensions, Ordian polybrachs are restricted to certain fixed numbers of arms corresponding to the faces (or vertices) of different platonic solids, of which there are only a finite number. The polybrachs have further specialized into three major clades based on their early embryonic development: tetrabrachs, cephalobrachs, and dodecabrachs.

In this figure, we can see the 3-or-fewer-dimensional stages of embryonic development from a single egg cell up to 4 or 8 cell structures, which allow the identification of different clades. Tetrabrachs (whose embryonic shape is labelled with a T in the preceding diagram) undergo only two cycles of cell division before adopting a maximally-dense tetrahedral arrangement of cells. The third cell division extends the embryo into the fourth vertical axis, with each tetrahedral segment going on to develop into a portion of the central disk and associated arm. Tetrabrachs tend to specialize in benthic habitats, like symmetrical flatworms, but are capable of much more active lifestyles.

Cephalobrachs (whose embryonic shape is labelled with a C) maintain a more open cellular structure through three divisions, producing a cubical arrangement of cells from which can develop eight distinct equally-spaced arms, corresponding to the faces of an octahedron. Their fourth cycle of division does not produce additional cells associated with an octahedral segment, though; rather, the top cube develops in an entirely different direction from the bottom of the creature, producing a glomular (4-dimensionally spheroidal) head / body cavity. similar to an Earthling cephalopod. Also like cephalopods, many species of cephalobrachs are capable of walking or dragging themselves along the ocean floor, but they are more often found in free-swimming niches.

Dodecabrachs (whose embryonic shape is labelled with a D) maintain an open square arrangement for two cycles of cell division, but then fall into  more close-packed square antiprism arrangement for their third. This third split already corresponds to the division between upper and lower body segments; a further cycle of division could establish cubical/octahedral symmetry, but that is not, in fact, what happens. Instead, several more cycles of cell division produce two joined spherical disks of cells, begin differentiating into distinct organs much later, eventually producing an arm section with either twelve segments in dodecahedral symmetry (hence the name of the clade) or, more rarely, twenty segments in icosahedral symmetry. The 12 vs. 20 choice seems to be easy to flip between as new species of dodecabrachs evolve, but there is a more fundamental division between sessile and medusoid dodecabrachs. In the sessile branch of the family, the body segment extends into a long spherinder (a sphere extruded into the fourth dimension, analogous to a 3D cylinder) which acts as a stalk to attach the animal to a solid surface, with the arms acting to filter nutrients from the water. In the medusoid branch, the body segment instead expands into a wide spherical disk. In some species, the disk remains relatively small such that the arms are free, and swimming is accomplished in a manner similar to an Earthling feather starfish; in most medusoids, however, the upper disk grows large enough to can curve around and enclose the central arm, disk rather like the bell of a 3D jellyfish, allowing jet propulsion by contracting the bell to expel water.

All polybrachs have ocelli (eyespots) at the ends of each of their arms, a feature which is believed to have been inherited from early flatworms before the two clades diverged; spherical flatworms also frequently have eyespots on their upper surfaces, in a variety of regular, semi-regular (corresponding to Archimedean solids) and random arrangements. Within the polybrachs, dodecabrachs appear to be the least-derived clade, with cephalobrachs and tetrabrachs each having split off from a dodecabrach ancestor after settling onto a power-of-two number of arms, which then permitted differentiation decisions to drift earlier in the stages of embryonic development.