The two-way crab is an intelligent alien with a strange form: It is biradially symetric, with multiple legs at the sides, 2 arms beside each face, and two faces at opposite ends of the body. It has no defined head. Each face has eyes, other sensory organs, a mouth, and a small internal ganglion. The brain is in the centre of the body. It has no shell or armour. Its body is roughly as wide as it is long. It can move in any direction with the same ease. Would this bodyplan work, and is it plausible that it could have evolved?
2$\begingroup$ Evolution on Earth doesn't favor two faces, so I think the real question is why does evolution on your aliens planet favor it. $\endgroup$– LenJun 18, 2021 at 17:12
$\begingroup$ so where does it excrete waste? $\endgroup$– JohnJun 19, 2021 at 21:22
$\begingroup$ One big problem, you have effectively halved the size of the digestive tract, the largest organ in most animals. you have drastically reduced (more than halved) the amount of nutrition it can extract from it's food. $\endgroup$– JohnJun 19, 2021 at 21:24
$\begingroup$ @John It could have a single digestive tract with two oesophagi $\endgroup$– Ichthys KingJun 19, 2021 at 21:47
$\begingroup$ @IchthysKing its still half the size because they are at opposite ends of the body. And again where is excreting waste? $\endgroup$– JohnJun 19, 2021 at 22:00
Radial symmetry is most commonly observed in organisms that move very little, if at all. The radial symmetry allows the organism to sense and interact with stimuli on any side of the body, without needing to move the body itself. For an intelligent, mobile creature like a crab, having a second face likely won't bring much benefit - since it is not a sessile creature, it can just turn to face nearby stimuli.
Body complexity is a liability, and is unlikely to arise unless there's a evolutionary benefit to doing so. What's the benefit to having a second face? It opens new ingress points for bacteria or infections, and the benefits are unclear. The crab can likely only feed one mouth at a time, and a wider range of vision can be achieved without needing a second face.
Given that there are currently no known creatures that have two faces, and very few mobile creatures that are radially symmetric, the natural evolution of such a creature in an earth-like setting seems unlikely. To make this more plausible, it should be clear what's the benefit of any of these features like a second face that cannot be achieved with just one.
2$\begingroup$ Starfish move a fair bit. Jellyfish move all the time! $\endgroup$– WillkJun 18, 2021 at 22:16
In case of two sets of eyes with a single brain, you could have a viable crab.
But I see a few other issues..
there is two vulnerable parts instead of one, predators would finish them off easily.
the eyes would be very primitive, because the connection to the retina will need to be relatively long. Normally the retina has a very short connection bundle to the brain.
internal competition between the two sets of eyes could arise, the animal would end up having a split brain, the motoric system will get disoriented.
no idea what happened on other planets, but on earth, animals developed from a tube shape, one side to take in food, the other side to get rid of the excrements. Having two mouths on either side would violate the basic design. The mutation would need to be huge. A second mouth would require developing some outlet elsewhere, at the same time ! The probability of two rigorous mutations taking place at the same time is very low.
$\begingroup$ A two-mouthed design could arise from the begining, for example if there is already some sort of predator or other such danger in the ocean when the first animaloids evolve, then it would make sense for an animal to be able to move in both directions, and so a biradial design would make sense $\endgroup$ Jun 19, 2021 at 21:10
$\begingroup$ At first, there are no preditors, it's surviving designs. Read Stephen Gould's books. Nature is experimenting with an optimal shape to do the basic things. The pre-Cambrian basic design was a very logical and minimal choice. You have a tube, or cylinder shape and you have food processed inside that shape. It seems sensible to have one mouth and one anus, residing on either side of the body. Any alternative would become more complex. On other planets, the initial design adapted by most species could e.g. be tripod (triangular), allowing to develop two faces and one outlet. $\endgroup$– GoodiesJun 20, 2021 at 12:10
$\begingroup$ Predators could exist in a world without animaloids, feeding on planktonic pre-animaloids, which then evolve muscles and nerves and such $\endgroup$ Jun 20, 2021 at 17:27
$\begingroup$ Of course, but in that case your predator itself would have been part of the initial shape experiments nature got involved in. When the predator is effective, the experiment succeeds. It's interesting you mention plankton.. in fact plankton is an important reason the tube shape works in Earth's water streams. Plankton goes through, the tube absorbs what it "eats". The alternative, a tripod shape would not be effective in case of plankton, because the mouths would not be placed in a good (or optimal) angle .i.r.t. the incoming water stream. A tube is the simplest solution for plankton-eaters. $\endgroup$– GoodiesJun 20, 2021 at 17:40