Questions 11-15 are based on the following pessage!

    Certainly no creature in the sea is odder than the common sea cucumber. All living creature, especially human beings, have their peculiarities, but everything about the little sea cucumber seems unusual. What else can be said about a bizarre animal that, among other eccentricities, eats mud, feeds almost continuously day and night but can live without eating for long periods, and can be poisonous but is considered supremely edible by gourmets?

    For some fifty million years, despite all its eccentricities, the sea cucumber has subsisted on its diet of mud. It is adaptable 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 nutrients 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 tentacles. Usually the creatures are cucumber - shaped - hence their name - and because they are typically rock inhabitants, this shape, combined with flexibility, enables them to squeeze into crevices where they are safe from predators and ocean currents.

    Although they have voracious appetites, eating day and night, sea cucumbers have the capacity to become quiescent and live at a low metabolic rate - feeding sparingly or not at all for long periods, so that the marine organisms that provide their food have a chance to multiply. If it were not for this faculty, they would devour all the food available in 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 crabs, and when attacked, it squirts all its internal organs into the water. It also casts off attached structures such as tentacles. The sea cucumber will eviscerate and regenerate itself if it is attacked or even touched; it will do the same if surrounding water temperature is too high or if the water becomes too polluted.

11. What does the passage mainly discuss?