Introduction 



To 



.O develop a really friendly feeling for a jelly- 

 fish or a flatworm takes a lively imagination. And 

 even to tell head from tail in many invertebrates one 

 also needs some information. This poses for the 

 writer on invertebrates special problems of presenta- 

 tion that do not arise in quite the same way in books 

 on the natural history of vertebrates. Show anyone 

 a vertebrate, even so lowly a one as a goldfish, and 

 he can immediately identify himself with it, for it 

 has the same "two-sided" or bilateral symmetry as 

 himself. He not only knows head from tail but back 

 from belly and right side from left. He knows where 

 to approach it with an offering of food, and which 

 end will go first when it swims away. Gazing into its 

 two symmetrically placed eyes, he does not doubt 

 that the fish is looking at him, and he may even im- 

 agine that his image evokes a psychological response 

 that is closely akin to his own feeling of relatedness. 



Not so with many of the lower invertebrate groups. 

 Their bodies may be spherical, as in many of the 

 floating protozoans, or they may be radial in sym- 

 metry, as in jellyfishes and corals. Even in bilateral 

 invertebrates like mollusks and insects, the legs may 

 wrap around the head, the multiple eyes may encir- 

 cle most of the body, or the ears may be mounted in 

 the legs. There are groups of invertebrates that su- 

 perficially are difficult to distinguish from seaweeds 

 and are almost as unresponsive. Many of the most 

 fascinating invertebrate groups require the use of a 

 hand lens or a microscope to be seen at all. Yet it is 

 the very strangeness of invertebrates — in contrast to 

 the relative sameness and predictability of the gen- 



erally four-limbed vertebrates — that attracts us so 

 strongly. Whether we are exploring the sea bottom 

 with an aqualung, eagerly following a receding tide, 

 or merely wading about in a brook, the constant ex- 

 pectation of coming upon some hitherto unimagined 

 living shape or some undreamed-of way of life is an 

 exciting challenge — but a challenge on a purely aes- 

 thetic or intellectual level. For there is little emo- 

 tional warmth to be derived from fondling a beauti- 

 ful jellyfish or a colorful crab. Though there is great 

 sensual enjoyment in the kaleidoscopic variety of in- 

 vertebrate shapes and color patterns, this has its 

 limits — even with animals as lovely or as bizarre as 

 are many of the invertebrates. 



The inexhaustible possibilities for intellectual en- 

 richment through contact with invertebrate animals 

 must come mostly through knowing something of 

 their habits, their distribution, their role in the nat- 

 ural communities in which they live, their variety of 

 structure, the basic relationships of even the most 

 seemingly diverse forms, their relative structural 

 complexity, and their origins in the grand scheme of 

 evolutionary history. The last four matters, it must 

 be added, can only be touched on in a book of this 

 kind. 



The authors hope only to give the reader, through 

 both text and photographs, some vicarious familiar- 

 ity with the external appearance of invertebrates 

 (excepting the insects) and some understanding of 

 their habits, their environmental adaptations, and a 

 few of the more interesting ways in which they en- 

 ter into our own lives. 



What is an hivertehrate? 



The word "invertebrate" is a semantic blanket 

 that covers most of animal kind and reveals nothing 

 of the varied shapes that have been thrust under it. 

 To lift one corner and glimpse a few of the more 

 familiar invertebrates — worms, starfishes, snails, 

 clams, crabs, and butterflies — is a mere beginning 

 toward appreciating a variety of creatures that range 

 in size and in complexity from microscopic proto- 

 zoans to giant squids 50 feet long, and that com- 

 prise 97 per cent of the nearly a million different 

 kinds of animals that scientists have so far described 

 and named. About 685,000 of the invertebrate spe- 

 cies are built very much alike and are grouped to- 

 gether as the class Insecta. They are treated in a 



separate volume in the series of which this book is 

 a part. 



To be called an invertebrate, an animal need have 

 no one special shape, nor any specific structure, nor 

 any single positive attribute. It need only, for lack of 

 a vertebral column or backbone, be excluded from 

 the select company of the vertebrates. All verte- 

 brates, including man. have down the middle of the 

 back a row of articulated bones or sometimes carti- 

 lages. Each of these pieces, called a vertebra, is 

 rigid; but since the vertebrae are movable upon one 

 another, they provide just that combination of high 

 tensile strength and flexibility needed to support the 

 large body size, the marked muscularity, and the 



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