These two layers are separated by a jelly-like material called mesoglea. Amoebocytes are present within the mesoglea. These cells obtain food molecules from collar cells. They then move to the epidermal cells and release food to these cells. Nitrogen waste and carbon dioxide leave sponge cells by diffusion into the water moving through the sponge.
Sponge cells obtain oxygen from water moving through the sponge by diffusion.
One group of sponges has a skeleton composed of carbon dioxide, another has one of composed of glass, and a third group has a skeleton composed of protein called spongin. One special feature of sponges is found in fresh water sponges. These organism from over-wintering structures called gemmules. The sponge dies as winter begins, and the gemmule survives through the winter. In spring, each gemmule forms a new sponge.