9781422275054

cover it. This grows and grows until eventu- ally a large pearl is formed. The immense variety of forms of shells is the result of millions of years of evolution. Although in our eyes some of the shells may seem to be unnecessarily adorned with spi- rals, projections, coils, or other structures, there is a good reason for them. During the course of evolution, structures that serve no function will not survive. Structures that make a mollusk less successful in its envi- ronment will probably lead to its failure to breed or early capture by predators, so only those mollusks that possess all the features necessary for survival in a harsh and uncom- promising world will live long enough to

edge by the mantle as the shell grows. Shells found washed ashore on a beach often lack this layer; the abrasive effect of sand and bleaching by the sun can lead to beached shells looking washed out and bearing little resemblance to the brightly colored living shells. The inner layer of the shell is often beautifully colored and smooth; this is the result of the presence of a thin layer of nacre, or mother-of-pearl. This provides a comfort- able environment for the mollusk to live in. The nacre will coat irritant objects that enter the shell, the best-known example being the pearls formed inside oysters. If a tiny grain of sand enters the shell, it will irritate the del- icate mantle tissues, so nacre is secreted to

An extremely rare deepwater Sthenorytis turbinum, or turban wentletrap, from the Galapagos Islands, the shell of the wentletrap

is constructed like a winding

staircase, with a descending spiral of raised ribs.

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