A sponge’s body consists of two layers of cells surrounding a gel-filled space. The body is supported by a tough protein mesh and, in most species, a number of hard spikes called spicules. The spicules are glasslike or made from limestone and vary in shape. The body of a sponge forms a network of water- filled passages and chambers. Sponges are very efficient filter-feeders. Water is sucked into the chambers and then forced out through separate openings.