Just below their skin is an endoskeleton composed of calcareous plates or ossicles.
From the en.wikipedia.org
Bones are rigid organs that form part of the endoskeleton of vertebrates.
From the en.wikipedia.org
The T-800 features living skin and tissue over a metal endoskeleton.
From the sacbee.com
This technique was used for the endoskeleton in The Terminator.
From the en.wikipedia.org
Inner solid pale circles show the unmineralized endoskeleton.
From the nature.com
Inner solid-dark circles show the mineralized endoskeleton.
From the nature.com
A true endoskeleton is derived from mesodermal tissue.
From the en.wikipedia.org
The tortoise has both an endoskeleton and an exoskeleton.
From the en.wikipedia.org
It has an endoskeleton of calcified ossicles and is covered in a fleshy layer of skin giving it a rubbery appearance.
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More examples
The internal skeleton; bony and cartilaginous structure (especially of vertebrates)
An endoskeleton is an internal support structure of an animal, composed of mineralized tissue. In three phyla and one subclass of animals, endoskeletons of various complexity are found: Chordata, Echinodermata, Porifera, and Coleoidea. ...
(Endoskeletal) A prosthesis composed of an internal pylon system which provides structural integrity to the device.
(Endoskeletal) A prosthesis which typically uses a pylon (tube) to support the weight of the amputee. This prosthesis is typically covered with soft foam and nylon.
An internal supporting skeleton with muscles on the outside; in vertebrates, consists of the skull, spinal column, ribs, and appendages.
Internal supporting bone framework of vertebrates
Some believe that the xenomorph contains an endoskeleton as well as an exoskeleton. The primary fuel for this debate is the xenomorph skull seen as a trophy in the movie Predator 2, but until AvP the two franchises were distinct, besides this nod to the Alien films. ...
A hard skeleton buried within the soft tissues of an animal, such as the spicules of sponges, the plates of echinoderms, and the bony skeletons of vertebrates.
END-oh-SKEL-uh-tuhn/ An internal skeleton, as opposed to an exoskeleton.