Definition of "endoskeleton"

This is the case not only with all forms provided with exoskeleta and endoskeleta, into the composition of which lime largely enters, but also with those which exercise the precipitating effect on the calcium salts they absorb from sea water, the precipitation rarely going so far as to form a distinct deposit in the cells or tissues of the organism.

1903 January 17, A[rchibald] B[yron] Macallum, “The Palæochemistry of the Ocean in Relation to Animal and Vegetable Protoplasm”, in Transactions of the Canadian Institute, volume VII, Toronto, Ont.: […] for the Canadian Institute by Murray Printing Company, published 1904, section IV (The Composition of the Primeval Ocean), page 551

Endoskeleta have the advantage that they can be flexibly “restructured” when the situation demands.

1978, K.H. Pribram, “Modes of Central Processing in Human Learning and Remebering”, in Timothy Teyler, editor, Brain and Learning, Stamford, Conn.: Greylock Publishers, section “The Implications”, page 156

The acantharian radiolaria can similarly collect and precipitate strontium sulfate to form their endoskeleta. […] Besides the axopods, most other radiolaria form endoskeleta of either strontium sulfate (the Acantharea) or, more often, opaline silicates (the Polycystinea and Phaeodarea).

1991, Eugene C. Bovee, “Sarcodina”, in Frederick W. Harrison, John O. Corliss, editors, Microscopic Anatomy of Invertebrates, volume 1 (Protozoa), Wiley-Liss, pages 205 and 213