Why can some demosponges be used as natural bath sponges?

Certain species of the class Demospongiae are used as natural bath sponges because of their soft, elastic skeletal frameworks.1

These sponges, particularly those in the order Keratosa, possess a skeleton composed entirely of spongin fibers. Spongin is a scleroprotein—similar to the keratin found in human hair and nails—that provides the structural flexibility and softness required for commercial use. Unlike other sponges that may contain rigid mineral spicules or inclusions like sand, these specific commercial sponges are prized because their spongin fibers are free of such inclusions, resulting in a texture that is soft and elastic.234

To prepare them for use, the living sponge—which is a mass of cells and fibers permeated by a canal system—must be thoroughly cleaned to remove all living cells, leaving behind only the durable, absorbent spongin framework.1