The first fish may have crawled onto land during Devonian times (350 million years ago) and probably did so, in response to drying swamps. This required that two problems must be overcome. How to move without the support of water and how to obtain oxygen from air rather than water. The mud skipper, another interesting denizen of the mangroves, has solved both these problems. Their generic name Periopthalmus (Latin=round eyes) was given because their eyes are perched on the top of the head and can be moved independently of each other. This gives the fish virtually 360 degree vision. Mudskippers can often be seen skittering over the mud searching for insects and small crustaceans to eat. Their front, upper (pectoral) fins are quite muscular and function rather like primitive legs, while their front lower (pelvic) fins are modified into a sucker disc. The sucker enables them to cling to the roots and trunks of the mangrove trees. This fish is able to survive out of water because its gills are housed within an enlarged cavity which contains both water and air. The tissue within the cavity can absorb oxygen from the air as long as it remains moist, so it functions like a kind of primitive lung. The mudskipper must return periodically to the sea, however, to replenish the water in its gill chamber.