It is vital to your understanding of this topic that you remember life on Earth was largely water-based and these aquatic creatures had to undergo adaptations and evolution over millions of years of transition to make the conversion to life on land possible. Animals had to develop lungs in addition to or replacing gills that could breathe air and extract the oxygen from that rather than the water. They also had to develop feet to replace their fins. Animals also had to be able to adjust to wide ranges of temperature on land that they didn't experience in the water, thus they developed different forms of heat protection. Mainly, animals had to develop a respiratory system that could handle the increase of oxygen, feet/legs that were suited to land rather than water, and stronger bodies that were rigid and could withstand high/low temperatures, terrestrial predators like those of vertebrates and arthropods, that were the animals first able to live on land.