In terrestrial environments, gravity places special demands on the cardiovascular systems of animals. Gravitational pressure can cause blood to pool in the lower regions of the body, making it difficult to circulate blood to critical organs such as the brain. Terrestrial snakes, in particular, exhibit adaptations that aid in circulating blood against the force of gravity.  

　　The problem confronting terrestrial snakes is best illustrated by what happens to sea snakes when removed from their supportive medium. Because the vertical pressure gradients within the blood vessels are counteracted by similar pressure gradients in the surrounding water, the distribution of blood throughout the body of sea snakes remains about the same regardless of their orientation in space, provided they remain in the ocean. When removed from the water and tilted at various angles with the head up, however, blood pressure at their midpoint drops significantly, and at brain level falls to zero. That many terrestrial snakes in similar spatial orientations do not experience this kind of circulatory failure suggests that certain adaptations enable them to regulate blood pressure more effectively in those orientations.  

　　One such adaptation is the closer proximity of the terrestrial snake’s heart to its head, which helps to ensure circulation to the brain, regardless of the snake’s orientation in space. The heart of sea snakes can be located near the middle of the body, a position that minimizes the work entailed in circulating blood to both extremities. In arboreal snakes, however, which dwell in trees and often assume a vertical posture, the average distance from the heart to the head can be as little as 15 percent of overall body length. Such a location requires that blood circulated to the tail of the snake travel a greater distance back to the heart, a problem solved by another adaptation. When climbing, arboreal snakes often pause momentarily to wiggle their bodies, causing waves of muscle contraction that advance from the lower torso to the head. By compressing the veins and forcing blood forward, these contractions apparently improve the flow of venous blood returning to the heart. Questions 18–25 refer to the passage above. 

　　18. The passage provides information in support of which of the following assertions? (应用) 

　　(A) The disadvantages of an adaptation to a particular feature of an environment often outweigh the advantages of such an adaptation.  

　　(B) An organism‟s reaction to being placed in an environment to which it is not well adapted can sometimes illustrate the problems that have been solved by the adaptations of organisms indigenous to that environment.  

　　(C) The effectiveness of an organism’s adaptation to a particular feature of its environment can only be evaluated by examining the effectiveness with which organisms of other species have adapted to a similar feature of a different environment.  

　　(D) Organisms of the same species that inhabit strikingly different environments will often adapt in remarkably similar ways to the few features of those environments that are common.  

　　(E) Different species of organisms living in the same environment will seldom adapt to features of that environment in the same way.   

　　19. According to the passage, one reason that the distribution of blood in the sea snake changes little while the creature remains in the ocean is that (细节) 

　　(A) the heart of the sea snake tends to be located near the center of its body  

　　(B) pressure gradients in the water surrounding the sea snake counter the effects of vertical pressure gradients within its blood vessels  

　　(C) the sea snake assumes a vertical posture less frequently than do the terrestrial and the arboreal snake  

　　(D) the sea snake often relies on waves of muscle contractions to help move blood from the torso to the head  

　　(E) the force of pressure gradients in the water surrounding the sea snake exceeds that of vertical pressure gradients within its circulatory system   

　　20. It can be inferred from the passage that which of the following is true of species of terrestrial snakes that often need to assume a vertical posture? (推论) 

　　(A) They are more likely to be susceptible to circulatory failure in vertical postures than are sea snakes.  

　　(B) Their hearts are less likely to be located at the midpoint of their bodies than is the case with sea snakes. 

　　(C) They cannot counteract the pooling of blood in lower regions of their bodies as effectively as sea snakes can.  

　　(D) The blood pressure at their midpoint decreases significantly when they are tilted with their heads up. 

　　 (E) They are unable to rely on muscle contractions to move venous blood from the lower torso to the head.   

　　21. The author describes the behavior of the circulatory system of sea snakes when they are removed from the ocean (see lines 17–20) primarily in order to (评价) 

　　(A) illustrate what would occur in the circulatory system of terrestrial snakes without adaptations that enable them to regulate their blood pressure in vertical orientations  

　　(B) explain why arboreal snakes in vertical orientations must rely on muscle contractions to restore blood pressure to the brain  

　　(C) illustrate the effects of circulatory failure on the behavior of arboreal snakes  

　　(D) illustrate the superiority of the circulatory system of the terrestrial snake to that of the sea snake  

　　(E) explain how changes in spatial orientation can adversely affect the circulatory system of snakes with hearts located in relatively close proximity to their heads   

　　22. It can be inferred from the passage that which of the following is a true statement about sea snakes? (推论) (A) They frequently rely on waves of muscle contractions from the lower torso to the head to supplement the work of the heart.  

　　(B) They cannot effectively regulate their blood pressure when placed in seawater and tilted at an angle with the head pointed downward.  

　　(C) They are more likely to have a heart located in close proximity to their heads than are arboreal snakes.  

　　(D) They become acutely vulnerable to the effects of gravitational pressure on their circulatory system when they are placed in a terrestrial environment.  

　　(E) Their cardiovascular system is not as complicated as that of arboreal snakes.   

　　23. The author suggests that which of the following is a disadvantage that results from the location of a snake’s heart in close proximity to its head? (细节) 

　　(A) A decrease in the efficiency with which the snake regulates the flow of blood to the brain  

　　(B) A decrease in the number of orientations in space that a snake can assume without loss of blood flow to the brain  

　　(C) A decrease in blood pressure at the snake’s midpoint when it is tilted at various angles with its head up  

　　(D) An increase in the tendency of blood to pool at the snake’s head when the snake is tilted at various angles with its head down  

　　(E) An increase in the amount of effort required to distribute blood to and from the snake‟s tail   

　　24. The primary purpose of the third paragraph is to (评价) (A) introduce a topic that is not discussed earlier in the passage  

　　(B) describe a more efficient method of achieving an effect discussed in the previous paragraph  

　　(C) draw a conclusion based on information elaborated in the previous paragraph  

　　(D) discuss two specific examples of phenomena mentioned at the end of the previous paragraph  

　　(E) introduce evidence that undermines a view reported earlier in the passage   

　　25. In the passage, the author is primarily concerned with doing which of the following? (主旨) 

　　(A) Explaining adaptations that enable the terrestrial snake to cope with the effects of gravitational pressure on its circulatory system  

　　(B) Comparing the circulatory system of the sea snake with that of the terrestrial snake  

　　(C) Explaining why the circulatory system of the terrestrial snake is different from that of the sea snake  

　　(D) Pointing out features of the terrestrial snake’s cardiovascular system that make it superior to that of the sea snake  

　　(E) Explaining how the sea snake is able to neutralize the effects of gravitational pressure on its circulatory system

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