A gemmule is a small asexual reproductive structure formed by freshwater sponges. The gemmule can withstand extremes in temperature and oxygen. They can even withstand periods of dessication.

This image of a Spongilla gemmule is from http://homepage.uab.edu/acnnnghm/BY255L/BY255L-Porifera.htm. It shows a sphere of cells encased in spicules and spongin.