Some kinds of animals form their bodies into a circle. In some cases, they do this to protect themselves, and in some cases they do it to be able to roll. Animals that do this include some caterpillars, tiger beetle larvae, myriapods, mantis shrimp, and salamanders. Other animals form into a ball instead. These animals include pangolins, hedgehogs, armadillos, the armadillo Lizard, isopods, the wheel spider, and fossilized trilobites. These animals may roll "passively", meaning they are pushed by gravity or wind, or "actively", meaning they push themselves by changing their shape. Tumbleweeds are a part of a plant that grows above the ground. Tumbleweeds break off from their roots and roll in the wind in order to spread their seeds.