SHELLS AND MOLLUSCOUS ANIMALS. 173 



rich hues of its inhabitant are seen glowing through 

 the clear blue water in the rays of a tropical rising 

 sun." Nor is the colour of these animals all that 

 is remarkable. They have a beautiful and won- 

 derful mechanism, offering continually objects of 

 interest, from the delicate net-work, or gills rayed 

 like flowers, which in some species serve the pur- 

 pose of lungs, and the strong fleshy foot by which 

 the cockle leaps up, to the tongue of the limpet, 

 which is two or three inches long, and armed with 

 hooked teeth, that serve as a rasp in fitting its 

 food for nutriment. 



The large class of Mollusca comprises not only 

 those animals which live in shells, but a great 

 number of similarly constituted animals, which are 

 destitute of this external covering. Their name, 

 given by Cuvier, and derived from the Latin 

 word mollis, soft, expresses the nature of their 

 substance, and the oyster and snail are familiar 

 examples of these soft fleshy creatures. Differing 

 greatly in different tribes, yet they have some 

 marked resemblances common to all. They have 

 no internal skeleton, are all of a fleshy consistence, 

 have no legs; and though their fins are often 

 spoken of, yet these are mere expansions of a 

 portion of their skin, and not separate appendages, 

 though by their means the mollusks glide through 

 the waters. The greater number among them 

 have shells, either consisting of one piece or valve, 

 like the periwinkle; or of two valves, like the 

 oyster; or of more valves, as in the barnacle. 

 When composed of two valves, the shell opens by 

 means of a strong elastic hinge, and is closed, at 

 the will of the animal, by means of muscles which 

 pass from one valve to another. They usually 



