Question

What structures do sponges possess that may prevent other animals from eating them?

Answer 1

The answer is spicules. These sharp-pointed structures are formed from calcium carbonate skeleton of the organisms.  They can also be formed from silica. They can be big (megascleres), or microscopic (microscleres). Also dependent on the number of axis on the spicules, they are classified as monoaxon, triaxon or polyaxon.

Answer 2

Sponges have spicules, small spiky elements, in their bodies,  made out of different minerals in different sponge species that serve as protection against predators.
Since sponges have no organized tissues, just layers of cells, these spicules stand between two layers of sponge cells.