托福阅读真题+题目+答案：Echinoderm Evolution

托福阅读真题+题目+答案：Echinoderm Evolution

The echinoderms are a phylum of invertebrate animals that includes starfish, sea urchins, and sand dollars. One feature that distinguishes echinoderms from other groups of organisms is their system of locomotion-the bottoms of echinoderms are covered with a number of tiny tube feet, which aid the echinoderm in feeding and slow movement on the ocean floor Like other invertebrates, echinoderms lack a backbone and have a fairly primitive nervous system. Unlike many invertebrates, the soft bodies of which are often poorly preserved as fossils, echinoderms have hard internal skeletons made of calcite. For this reason, echinoderms are well represented in the fossil record. In spite of their generally good preservation as fossils, there are several puzzles surrounding the evolution of echinoderms.

1.According to paragraph 1 ,what is one reason that many echinoderms were preserved as fossils?

O Echinoderms do not have backbones.

O Echinoderms have many small body parts that can be easily preserved.

O Echinoderms have hard structures inside their bodies.

O Echinoderms have feet that allow them to move across the seafloor.

One major mystery surrounding echinoderms is how they originated. The first clearly recognizable echinoderms appeared around 540 million years ago during the Cambrian period; however, these first echinoderms were already fairly complex. Paleontologists have been unable to conclusively identify a simpler organism that could have been the ancestor of the echinoderms. One fossil that has been tentatively suggested as an ancestral echinoderm is Tribrachidium, which dates to the Ediacaran period just before the Cambrian. Tribrachidium is round,flat,disc- shaped fossil that was discovered in 1959. Some paleontologists have suggested that Tribrachidium could have been a primitive echinoderm, as it shares the radial body plan of modern echinoderms. A radial body plan is one in which an organism's body is composed of parts that are arranged symmetrically around the center like the petals of a flower. Since most known echinoderms have radial body plans, the radial symmetry of Tribrachidium has been viewed as suggestive of an evolutionary relationship. However, Tribrachidium has now been discounted as an ancestral echinoderm since it has triradial (three-point)symmetry instead of the pentaradial (five-point)symmetry found in adult echinoderms. In 1985 Tribrachidium was placed in its own phylum, Trilobozoa, a group with no known living descendants.

2.Which of the following best describes the relationship of paragraph 2 to paragraph 1?

O Paragraph 2 provides an explanation for the unusual physical characteristics of echinoderms introduced in paragraph 1.

O Paragraph 2 explains some of the differences among types of echinoderms introduced in paragraph 1.

O Paragraph 2 gives an example of one of the puzzles about echinoderm evolution mentioned in paragraph 1.

O Paragraph 2 presents additional information about the similarities between echinoderms and other invertebrates discussed in paragraph 1.

3.It can be inferred that scientists believe which of the following about the"first clearly recognizable echinoderms"?

O They could actually be misidentified Tribrachidium fossils.

O They became fossilized during the Ediacaran period.

O They had different body plans than modem echinoderms.

O They are too complex to be the earliest form of the animal.

4.4.According to paragraph 2, why do some scientists believe that Tribrachidium may not actually be an ancestor of the echinoderms?

O It appears at the same point as echinoderms in the fossil record.

O Its body plan is not the same as that of adult echinoderms.

O Its body parts are arranged symmetrically around a center.

O It is more primitive than paleontologists would have expected an echinoderm ancestor to be.

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