Sponges are among the world's simplest organisms, having no brain, nerves or muscles. Conversely, they have some of nature's most complex individual cells. Sponge spicules (cells) belong to three different groups: bony, glass or spongy, depending on their structure.
Sponge cells viewed through a loupe
Scanning Electron Microscopic views of sponge spicules Building Blocks of sponges