I thought you said "testicle monsters" and was wondering where you got testicles then I saw the pouches on the crotch then realized you said "tentacle"...
They inject a kind of silicon into the bloodstream which hardens, showing the blood vessel system, at least thats how they did it to the pigs we dissected in my bio class
This is the best argument for evolution. Look at all the different parts that we share from other species, seeming to differ in a pattern from the fossil records!
I'm sorry.
Inject acid-resistant material
Let material set
Dissolve body in acid