In the book The Host by Stephenie Meyer, a parasitic species (Souls) attach to the back of the spinal cord of alien creatures to control their brains, but I find this a wee bit unbelievable. As it is unlikely that a wide range of aliens would have a spinal cord, what feature of the nervous system is likely to be found across a wide variety of alien species?
First of all, this is not science fiction. Read up the life cycle of the sheep fluke, which at one stage takes over the brain of an ant to ensure it can be eaten by a sheep. Currently, there's a journalistic panic about toxoplasma in undercooked meat taking over human brains to make them turn themselves into cat food (I kid you not). This sounds like something made up by the soybean industry.
But to get to the point:
- Some alien animal life is certain to develop bilateral symmetry. This is a winning design on earth, and has arisen independently among insects and vertebrates.
- Bilateral symmetry requires a segmented body, each segment having a specialized function, with its own neural controls.
- The front end must have sensory organs, if only to find food.
- Sensory organs require neural coordination and control.
- This predisposes to the development of an important neural collection at the front end.
- This collection will have to communicate with the rest of the body, if only to keep it moving towards the food.
- This means that nerve tracts will have to extend from the front backwards.
- If this animal lives on a planet with minerals, it will develop a skeleton, if only to segregate the minerals it can't use and can't get rid of. And which planet doesn't have minerals?
- This skeleton will evolve to protect the squishy nerves.
- So rather than being unlikely, a spinal cord with a bony shell seems almost an almost inevitable feature of a sufficiently complex organism.
Of course, we also have asymmetrical animal life forms on earth, the most successful being the octopus. But having no skeleton, the octopus is confined to the sea, leaving the land free for those other animals with the spinal cords.
Trying to get a handle on this question, I took a look at Wikipedia to see what a nervous system actually does:
At the cellular level, the nervous system is defined by the presence of a special type of cell, called the neuron, also known as a "nerve cell". Neurons have special structures that allow them to send signals rapidly and precisely to other cells.
It sends signals. That is the truly universal part.
They send these signals in the form of electrochemical waves…
Electrochemically. While that aspect is not absolutely essential, surely this method is likely to be among the fastest sorts of signalling that could evolve in a physical body. "Electrochemical signalling" is the closest thing I can offer to an answer to your question as currently formulated. However I think your question may include an overly-limiting assumption.
…traveling along thin fibers called axons, which cause chemicals called neurotransmitters to be released at junctions called synapses. A cell that receives a synaptic signal from a neuron may be excited, inhibited, or otherwise modulated. The connections between neurons can form neural circuits and also neural networks that generate an organism's perception of the world and determine its behavior.
Junctions. Modulations. Connections. All of this is where Information Theory meets Neuroscience, neither of which I know much about. But, along with the concept of signalling itself, isn't information processing, rather than any material property, the rock-bottom essential property of consciousness that the Soul or other Invasion of the Body Snatchers type of mental parasite (or demonic possessor, or Granny Weatherwax) must hitch on to?
So as well as, or instead of, a physical form, our parasite/symbiote exists in… er... information space or something and… er…
OK. That's really lame. But something like that.
As the first paragraph of the excellent answer by "frank" above says, real life evolution came up with something as weird as the lifecycle of the various sorts of fluke. Perhaps if we could describe how a being would evolve to take over the bodies of multiple drastically different species from different planets, we would be able to better theorize on how exactly it does it.
Likely to be found across a wide range of "animal-like" species: a central controller and head comprising a sensory cluster and feeding orfaces.
The "brain" needs to send and receive signals across the rest of the body. A central cord might not be the only way to do it: look at a starfish or octopus.
There have been discussions here about multiple brains or distributed systems. But they still connect to each other and to everything else.
same: signal distribution with a hub.
don't assume the same: the single-run topology.
same: some kind of cables carrying signals.
variety: how they work. Will your mechanism be able to tap in?
It depends on the type of planets that the aliens come from. If it the planets are similar to earth then having alien creatures with spinal cords is likely, as most creatures on earth have spinal cords.
Having read Host I don't think that they (Souls) only inhabit creatures with spinal cords. Wanda talks about living in a "seaweed" type creature.
The souls probably have multiple ways of talking over a creature's body. Attaching to the spinal cord is just the way that they take over human bodies. They probably use a completely different way when dealing with other creatures.
Unfortunately, aliens being alien, there are so many possible ways that signals could be transmitted around an organism that the souls couldn't realistically have a hope of being able to interface with them all. One might use electrical, the other might use chemical, the third might use light, and so on.
The souls are a scientifically implausible species. Magic must be involved.