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.

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

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).