In the sci-fi setting that I've had on the backburner for a while now, there is a species known as the Azmarans. They are a very socially adept species, between them having a rather communicative psychology to their unique ability to conform to whatever shape they might need in dealing with another species.
How they do this is through the use of artificial, inorganic exoskeletons, which are designed based on whatever bodily arrangement they so desire. They can also change their exoskeletons like a hermit crab would change their shell once they've grown too large for it. But for the Azmarans, this is more to achieve certain physical characteristics for social or practical benefit. For example, if they wanted to deal with a dog, they would make a dog-shaped exoskeleton, meanwhile if they wanted to deal with a human, then they would merely remove themselves from the dog exoskeleton and reinsert themselves into a human-shaped exoskeleton.
The way I'm imagining it is that their anatomy and the arrangements of their internal organs shifts and changes so that they can fit into whatever exoskeleton they want to. They have no mouths, as their diet consists of large amounts of bacteria and other microbial lifeforms, they are very sensitive to sound when naked, due to the atmosphere of their evolutionary homeworld having been rather thin, and they have a set of eyes developed for ultraviolet light, though they can perceive other wavelengths through the aid of their technology--which is often integrated into their exoskeleton's visors.
But something I can't quite wrap my head around is; how do they change their anatomy so that they can conform to the shape of their exoskeleton? They have no endoskeleton, that's for sure, and I'm wanting to make them a single organism rather than a symbiotic colony of smaller lifeforms like the Lekgolo from Halo, but beyond that I'm unable to figure it out. How might such a species develop their biology in such a way that their organs, veins, arteries, and so on would shift and reshape for the shape of their exoskeleton?
[EDIT] This is distinct from the other question because the Azmarans are supposed to be able to take on any selection of limb numbers and structures (10 limbs, 100, 1,000, etc.) and they are generally rather formless without an exoskeleton; resembling more of a mass of living flesh with eyes than a true animal when they don't have an exoskeleton. The idea of the Azmarans being many smaller intelligent lifeforms is tempting, but for your information, my brother had already introduced me to the Portuguese Man O' War, which is more of a colony of unicellular organisms functioning as one larger organism. This, however, is not what I'm looking for, as they are supposed to be distinct multicellular organisms. Not to menton that they are also supposed to be able to take the form of only what the exoskeleton is. That they use metamorphosis for this is out of the question, as they are expected to change their shape and physique depending on the shape of their exoskeleton.
I've also seen the comments regarding their size and whether I'm looking for that to change as well. I'm not. I'm wanting them to be stuck to their own regular mass and volume in what is possible, so if they were to take a mouse-type exoskeleton, then they would look like a massive mouse in their silhouette. Though the use of silhouettes that an organism would be comfortable with that are other than the organism's own is certainly a good idea, such as the dog issue; although simply being a large dog would typically suffice, they'd just as likely take a humanoid appearance, as dogs had been bred and raised to love humans and do what their human master says. In addition, I'm certain that humans wouldn't mind interacting with a humanoid of larger or smaller size, as the Azmarans would have different sizes depending on factours and variables similar to how size works for humans; age, nutrition, health, genetics, and the environment.