The effects of gravity on many-celled organisms is considerably more drastic. During the period when animals first evolved to survive on land some method of directed locomotion and thus a form of inner skeleton or outer skeleton would have been required to cope with the increase in the force of gravity due to the weakened upward force of buoyancy. Prior to this point, most lifeforms were small and had a worm- or jellyfish-like appearance, and without this evolutionary step would not have been able to maintain their form or move on land.