>>invertebrate<<

Perhaps not -- (from Webster's unabridged):

2. The elastic fibrous skeleton of many species of horny Spongi[ae] (keratosa), used for many purposes, especially the varieties of the genus Spongia. The most valuable sponges are found in the Mediterranean and the Red Sea, and on the coasts of Florida and the West Indies.

Apparently a skeleton need not be calciferous (?)

following the 'keratosa' clue:

\Ker"a*tin\, n. [Gr. ke`ras, -atos, horn.] (Physiol. Chem.) A nitrogenous substance, or mixture of substances, containing sulphur in a loose state of combination, and forming the chemical basis of epidermal tissues, such as horn, hair, feathers, and the like. It is an insoluble substance, and, unlike elastin, is not dissolved even by gastric or pancreatic juice. By decomposition with sulphuric acid it yields leucin and tyrosin, as does albumin.

missing link to:

scleroprotein: a simple protein found in horny and cartilaginous tissues and in the lens of the eye.

Apparently, a skeleton can consist only of cartilage.

Dr. Bill?