





Many invertebrates found in ttie sea - ttie 

 jeilyfish, for one - are absent from the iand. 

 Ttie Portuguese man-of-war is a "community" 

 of speciaiized polyps: some for feeding, otiiers 

 for reproduction, and others form stinging 

 tentacles that paralyze prey. All are attached 

 to the floating bladder, which was the 

 original larva from which the polyps budded. 



There are probably two million different kinds of plants and 

 animals in the world, at least. Yet, although two thirds of our 

 planet's surface is covered by the oceans, far fewer species of living 

 things dwell in the sea than on land or in the air. Of the roughly 

 one million species of insects, for instance, only half a dozen are 

 truly marine. And the more highly developed and larger plants live 

 almost exclusively on the land, leaving only seaweeds and the 

 microscopic plants of the plankton in the seas. There are no marine 

 ferns or mosses, no subaquatic trees. 



Marine biology — the science devoted to plants and animals that 

 live in the sea- ought, therefore, to be easier to study than the rest of 

 biology. But there are good reasons why this is not so. The most 

 obvious is that man himself is a land animal. Even as children, we 

 become familiar with the living things around us and learn about 

 them almost unconsciously. Throughout our lives we can continue 

 to investigate many land plants and animals without special effort. 

 We can more readily appreciate their way of life because they 

 breathe the air we breathe and because they respond to night and 

 day, to heat and cold, to the changing seasons, much as we do. 

 Even when they Uve in a far different environment from our own, 

 we can visit and study them easily; or they can be brought from 

 distant lands to a local botanical or zoological garden. And those 

 that are not available to us locally we can see in films and photo- 

 graphs. 



The sea is different. It is an alien world whose mysteries can be 

 probed only slowly and painstakingly. Hence our knowledge of 

 life in the sea has come late in the records of human history, and 

 we still have a great deal to learn. Another obstacle is that sea 

 plants and animals do not, for the most part, resemble those found 

 on land. Although there is a certain amount of overlapping, the 

 marine part of the living world is the more primitive and therefore 

 the less easy for us to understand because we do not always have 

 familiar standards for comparison. 



This point is made clear if we consider the two great subking- 

 doms of the animal world : the vertebrates (animals with backbones) 

 and the invertebrates (without backbones). If we ignore the over- 

 whelmingly large number of species of fish, we can regard the 

 vertebrates as mainly land animals that include those most familiar 

 to us - the mammals, birds, and reptiles. Only a few of the remain- 

 ing vertebrates - for instance, whales, seals, sea birds, sea snakes - 

 inhabit the oceans. The typically marine animal, then, again ignoring 

 the fishes, is an invertebrate. 



Several classes of invertebrates, like the sponges, sea anemones, 

 jellyfishes, and starfishes are wholly absent from the land, and very 

 few of their members live in rivers or lakes. Others, such as worms 

 and molluscs, are preponderantly marine, though they have a 

 number of relatives living on land. There remains the largest and 

 most highly evolved group, which can be conveniently described 

 as the higher invertebrates. They are the Arthropoda, or jointed- 

 legged animals, that include crustaceans, insects, spiders, milUpedes, 

 and centipedes. Except for a handful of species, the last four of 

 these are entirely terrestrial or live in fresh water. Together they 

 number more than a million species - over half the total of known 

 species of living organisms. 



Because of the numerical superiority of the land-living arthro- 



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