Certainly no creature in the sea is odder than the common sea cucumber. All living creature, especially human beings, have their peculiarities¹, but everything about the little sea cucumber seems unusual. What else can be said about a bizarre animal that, among other eccentricities², eats mud, feeds almost continuously day and night but can live without eating for long periods, and can be poisonous but is considered supremely³ edible⁴ by gourmets⁵??For some fifty million years, despite all its eccentricities, the sea cucumber has subsisted⁶ on its diet of mud. It is adaptable⁷ enough to live attached to rocks by its tube feet, under rocks in shallow water, or on the surface of mud flats. Common in cool water on both Atlantic and Pacific shores, it has the ability to suck up mud or sand and digest whatever nutrients⁸ are present.?Sea cucumbers come in a variety of colors, ranging from black to reddish?brown to sand?color and nearly white. One form even has vivid purple tentacles⁹. Usually the creatures are cucumber?shaped—hence their name—and because they are typically rock inhabitants, this shape, combined with flexibility¹⁰, enables them to squeeze into crevices¹¹ where they are safe from predators¹² and ocean currents.?

　　Although they have voracious¹³ appetites, eating day and night, sea cucumbers have the capacity to become quiescent¹⁴ and live at a low metabolic¹⁵ rate—feeding sparingly or not at all for long periods, so that the marine¹⁶ organisms that provid their food have a chance to multiply. If it were not for this faculty¹⁷, they would devour¹⁸ all the food available in a short time and would probably starve themselves out of existence.?

　　But the most spectacular thing about the sea cucumber is the way it defends itself. Its major enemies are fish and crabs¹⁹, when attacked, it squirts all its internal organs into water. It also casts off attached structures such as tentacles. The sea cucumber will eviscerate²⁰ and regenerate²¹ itself if it is attacked or even touched; it will do the same if the surrounding water temperature is too high or if the water becomes too polluted.?

　　20.  According to the passage, why is the shape of sea cucumbers important??

　　A. It helps them to digest their food.?

　　B. It helps them to protect themselves from danger.

　　C. It makes it easier for them to move through the mud.?

　　D. It makes them attractive to fish. ?

　　21.  The fourth paragraph of the passage primarily discusses ?____?.?

　　A. the reproduction of sea cucumbers?

　　B. the food sources of sea cucumbers?

　　C. the eating habits of sea cucumbers?

　　D. threats to sea cucumbers‘ existence ?

　　22.  What can be inferred about the defence mechanisms²² of the sea cucumber??

　　A. They are very sensitive to surrounding stimuli²³.?

　　B. They are almost useless.?

　　C. They require group cooperation.?

　　D. They are similar to those of most sea creatures. ?

　　23.  Which of the following would NOT cause a sea cucumber to release its internal    organs into the water??

　　A. A touch.  B. Food.?

　　C. Unusually warm water.  D. Pollution.