Fiber makers
Sponges are animals that live in water and stay stuck in one spot. Some sponges, like bath sponges, have a skeleton made of soft, bendy fibers instead of hard spikes.
Those fibers are made of a tough material called spongin. The cells that make it are called spongocytes (say "SPUN-joh-sites"). They line up in groups along a fiber and add more spongin to it.
They are a bit like the cells in your skin that make collagen, the stretchy stuff that helps hold your body together.
Wow!
Long ago, before plastic kitchen sponges, people used the dried skeletons of real bath sponges for washing. What they were holding was the spongin fibers!