Skeletal Diagram:
Though only male Twizzles can manipulate their tentacles, both genders have a series of them within their bodies that harden and act as a surrogate ribcage when internalized, protecting their organs.

Aside from that, the skeleton is pretty much similar to the average human.

Internal Diagram:
The Twizzle has no liver, and instead relies on three "filtering" hearts to keep its blood pure. Two hearts beneath the lungs circulate and filter blood to the third and smallest heart between the lungs, which can't handle Twizzle poisons and antitoxins normally.

For males, the two larger hearts begin to fail from constant contact with toxins and eventually begin to allow poison to reach the third heart, which slowly wears him down and kills him.

- Referenced from human skeleton and organ diagrams.