They Swim The Seas: The Mystery of Animal Migration
The watery world is in many ways similar to the mixture of gases we call the atmosphere Water has winds in the form of currents and there are fronts between warm and cold water just as there are between warm and cold air masses Currents flow up and down as well as side to side Freshwater animals living in rivers and lakes are also affected by currents though currents in small lakes are usually much weaker than those in huge seas Fish are more abundant in the earth’s waters than any other good sized animals They are found all over the world in salty oceans and seas and in freshwater lakes rivers streams ponds and swamps In fact there are as many different species of fish as there are of mammals birds reptiles and amphibians put together
Fish usually have gills that allow them to breathe the oxygen dissolved in water They are also cold-blooded which means their body temperature is nearly the same as the temperature of their surroundings The bodies of most fish are covered with scales a layer of protective bony or horny plates in overlapping rows Some fish such as eels have scales that are very tiny and difficult to see Most fish have streamlined shapes that allow them to move through the water rapidly The rear end of the body moves from side to side pushing the fish through the water The fins act as stabilizers and fine tune the movements allowing the fish to move up and down as well as side to side and forward and backward
Plants need water to live and many of them also journey in water Nonflowering plants such as seaweeds are often broken or torn by waves along shores and their pieces are carried away by ocean currents to grow in different places Freshwater nonflowering plants such as an alga called spirogyra also break apart and are spread by the moving waters of streams and ponds
Flowering seashore plants often have seeds that float and are protected from salt water by a tough coat or shell The seeds sprout when they come to rest on a warm sandy shore One such plant the beach morning glory grows throughout the Tropics along the high tide line The coconut palm produces huge nuts that are covered with a fibrous husk which helps to protect the nuts and also allows them to float in seawater The nut is really a seed and when it is washed up on a distant sandy beach it may germinate and give rise to a new palm tree
