There are approximately 800,000 species of invertebrates in the world. They are found virtually everywhere; where you find life you most probably find an invertebrate of some form or other. They are one of the eariest live forms to have walked the earth, and will undoubtedly be one of the last. Their sheer diversity ensures that whatever the environment there is a species that can survive in it: Antarctica; forgotten caves sealed off from sunlight in New Zealand; the bottom of the Pacific Ocean; in such diverse locations you will find invertebrates.
Invertebrates come in almost any form imaginable, from microscopic organisms which are all around us and which we can't see, to massive 2 ft stick insects. Their diet is so also extremely diverse, in fact there is a species to eat virtually anything, from leaf eating stick insects to bird eating tarantulas (although this is a rare occurance, it does happen). There are also scavenging species such as cockroaches, which will eat virtually any foodstuffs, attained in any way (hunted, grazed, scavenged, etc).
Invertebrates cannot be classified into one body design - in fact you can have legless worms to millipedes which can have hundreds of legs. You can have spiders with two body segments (the thorax and the abdomen) and you can have millipedes with up to a hundred body segments.
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