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第1段
1 .<-NARRATOR:->Now listen to part of a lecture on the topic you just read about.
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第2段
1 .<-FEMALE PROFESSOR:->Unfortunately, each of the three theories about how agnostids lived has a serious weakness.
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第3段
1 .First, we know that other types of arthropods swam in the open ocean, hunting their prey.
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2 .However, all of those arthropods had large, well-developed eyes.
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3 .Vision is one of the best ways for a predator to track its prey.
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4 .But agnostids had tiny, poorly-developed eyes and were sometimes completely blind!
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5 .This seems to rule out the idea that they were predators.
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6 .If they did chase after prey, they would have had some other, special sensory organ to help them find prey, but there's no evidence of this in the fossil record.
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第4段
1 .Second, it seems unlikely that agnostids lived on the seafloor.
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2 .Animals that are seafloor dwellers typically don't have the ability to move very fast or very far.
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3 .They move slowly across the seafloor and stay in localized areas rather than spreading to new areas.
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4 .So,typically we find each seafloor dweller species occupying a small geographic area where it had originated and nowhere else.
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5 .However, many agnostid species inhabited multiple geographic areas spread across large distances.
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6 .This suggests that agnostids were able to move from one area to another pretty fast.
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7 .The ability to move easily across large distances would be highly unusual for seafloor dwellers.
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第5段
1 .Finally, the parasite theory.
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2 .Well, one thing that's typical of parasites is that their populations are not very large.
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3 .Parasite populations have to stay within certain limits, because if there were too many parasites, they would kill off the host organisms they live on.
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4 .But we're pretty sure that the populations of many agnostids were in fact very large.
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5 .We can tell because for many species we've been able to find vast amounts of fossilized individuals.
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6 .So, the great size of agnostid populations seems to rule out the theory they were parasites.
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