A Tale Of Two Symmetries

(While this is a retcon, a lot of my older ideas suck, and I feel like this new change may benefit the project as a whole. Retcons are a process of worldbuilding.)

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(1 = Radial, 2 = Neo-Bilaterian)

Ithlo, the home planet of the Shjaum, is a world dominated by so-called “bulb walkers,” aptly named due to their body plan, in which the mantle of some representatives can have a bulbous shape. Bulb-walkers come in many different varieties and colors, and representatives belong to one of two groups: radials, and neo-bilaterians.

All terrestrial bulb-walkers can trace their ancestry to small, radially-symmetrical organisms that came onto land shortly after plant life successfully colonized dry land. These ancient forms had an internal structure similar to a skeleton, but it was nowhere near as rigid as modern bulb-walker skeletons. While the water was able to support such a structure, this would not cut it for land. So, very quickly, these radially-symmetrical colonizers developed a true, rigid skeleton, but did so independently, and the “direction” of their limbs directly influenced what niche they would fill. This is also where radials and neo-bilaterians come in.

Radials are, as their name may hint, radially-symmetrical bulb-walkers. They are defined by legs that bend outward. This provides them with a stable base, and the ability to effectively move in any direction. However, the tradeoff for a steady base and omnidirectional movement is their slow, ponderous nature. They rely either on size or horn-like ornamentations to keep themselves safe from predators. Curiously, radials tend to be larger on average than neo-bilaterians, and that comes down to how their legs behave. Radial legs are made up of multiple, elongated bones, all of which are rigid and columnar. Additionally, because they are not optimized for speed, their legs typically have fewer joints and points of articulation. 

Neo-bilaterians, on the other hand, are mostly defined by a pair of front-facing legs that bend inward, while their rear legs are semi-outward-bending. This somewhat more complicated limb orientation allows for quick movement. Because the front legs bend inward, they carry forth the entire creature, while their back legs provide stability and balance. Their legs also contain more joints and points of articulation—perfect for their highly-mobile lifestyle and their front-facing nature. 

In essence, radial legs are built for load-bearing, while neo-bilaterian legs are built for efficient locomotion.

Another way in which these classifications differ is in the location of their feeding trunk. All bulb-walkers feed through a trunk. For radials, their trunk is located between their four legs. For neo-bilaterians, their feeding trunk is positioned forward, between its front limbs. 

Perhaps the most interesting difference is how they understand reality. Radials retain the decentralized nervous system of their ancestors. Neo-bilaterians possess a centralized nervous system.

Radials perceive the world through their entire body at once because of their retained decentralized nervous system. This means they are aware of everything, but lack a clear focus. Even when focusing their attention onto a predator, the predator doesn’t necessarily gain all of the creature’s focus, but has emphasis placed on it. 

With neo-bilaterians, they perceive the world through their centralized brain located in their mantle. The brain is fed information constantly through the forward-facing eyestalks, hearing antennae, and nostrils. This results in them having a highly-efficient system of focus and priority, at the cost of being able to see in all directions.