Collagen is the protein that all animals have in common, even single-celled animals and creatures such as sponges that lack differentiated bodies. Collagen is a cable-like protein that can calcify into bone, shell or other hard structures. It mostly serves as connective tissue between animal cells.
The majority of protein in the human body is collagen, and it is also found abundantly in the bodies of other animals. Collagen holds the various tissues making up animal bodies together. It is flexible yet tough and strong. A similar role is played by pectin in plants and fungi.
Although sponges do not contain tissue in the same way that humans or fish do, their skeletons are made up of collagen. This porous collagen skeleton is the part of the animal that humans use to absorb water.