Endoskeletal: relating or referring to the endoskeleton.
"Explanation of Terms Used in Entomology" by John. B. Smith
Apparently have endoskeleton, but organization of the internal organs remains obscure.
"The Native Soil" by Alan Edward Nourse
The alien had an endoskeleton, but he also had a tough hide that was somewhat like thick, leathery chitin.
"Anything You Can Do ..." by Gordon Randall Garrett
The skeleton of the Mammalia consists almost solely of the endoskeleton.
"The Cambridge Natural History, Vol X., Mammalia" by Frank Evers Beddard