Figure 31.18 Arthropod Relatives with Unjointed Appendages (A) Tardigrades (water bears) can be abundant on the wet surfaces of mosses and plants and in temporary pools of water. (B) Onychophorans (velvet worms) have unjointed legs and use the body cavity as a hydrostatic skeleton. They are sometimes referred to as “living fossils,” meaning they are an ancient group that has changed very little over millennia.