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第一段
1 .Unfortunately, each of the three theories about how agnostids lived has a serious weakness.
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第二段
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, welldeveloped eyes, vision is one of the best ways for a predator to track its prey.
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3 .But agnostids had tiny, poorly developed eyes, and were sometimes completely blind.
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4 .This seems to rule out the idea that they were predators.
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5 .If they did chase after prey.
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6 .They would have had some other special sensory organ to help them find prey.
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7 .But there's no evidence of this in the fossil record.
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第三段
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 seafloordweller species occupying a small geographic area where it had originated and nowhere else.
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5 .However, many agnostids 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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第四段
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.
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4 .Because if there were too many parasites, they would kill off the host organisms they live on.
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5 .But we're pretty sure that the populations of many agnostids were in fact very large.
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6 .We can tell because for many species we've been able to find vast amounts of fossilized individuals.
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7 .So the great size of agnostid populations seems to rule out the theory that they were parasites.
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