Animals Move Out of the Sea
ONCE PLANTS AND FUNGI started invading the lands some 400 million years ago, new pastures became available for animals to exploit. These new opportunities did not long stay neglected. Modifications that had been of no use to aquatic animals in earlier days now became advantageous in the changed surroundings. What water-adapted animals mainly needed in order to take advantage of the rich new sources of food offered by land plants was to be able to resist loss of water, to utilize atmospheric oxygen (respiration), to move on land (for those that lacked such means), and to reproduce away from water. These adaptations were gradual and occurred first on coastal fringes and in marshy areas still exposed to intermittent flooding. Most aquatic animals, except for the lower invertebrates, developed solutions of one sort or another to the problems of life on land. I shall consider only two types of animals, the arthropods and the vertebrates, which together account for the major part of the terrestrial fauna.
Arthropods had it easiest, being already shielded by a waterproof covering and equipped with functional legs. However, their fragile gills could not long have resisted desiccation. What helped arthropods to utilize oxygen was the formation of thin, tubular invaginations of their carapace. These tenuous air ducts, or tracheae, progressively developed into a highly ramified network of passages that penetrated all parts of the body, allowing them to be in close contact with outside air. The thin