Because it has one valve cemented to the rock; and though the oyster itself has a heart, blood-vessels, brain, gills, and stomach, it depends on the bounty of the waves for all the objects of its sensation and nourishment. It is, however, an admirable provision of nature, that although the oyster and other natives of the water are thus stationary themselves, the fluctuations of the element in which they live, produce a variety in the scene, and daily bring new objects in contact with their organs of sensation.
Of oysters there are several species. Thus, in a little Manuel de l' Amateur d'Huitres, before us, we count upwards of forty-five different kinds which are known to naturalists.
Because it may participate and enjoy the returning tide. This is done, as well as the shell closed, with prodigious force, by means of a strong muscle at the hinge.
Mr. Carew, in his Survey of Cornwall, 1602, tells us of an oyster having opened its shell, and three mice attempting to seize it; but the oyster clasped fast its shell, and killed them all.