310 THE BRITISH NATURE BOOK 



26. The Pelican's Foot (Aporrhais pes-pelicani) , about i inch long and very 

 thick, has its shell expanded into a sort of lip or lobe, giving a fancied resem- 

 blance to the webbed foot of a water fowl. 



27. Pheasant Shell (Phasianella pullus) is very small (J inch), and must 

 be sought for carefully, but is quite common. It is smooth, white, or pale 

 yellow, thickly covered with fine crimson lines. 



28. The Hungarian Cap (Pileopsis hungaricus) is a brownish or whitish 

 shell, which has a kind of curved beak, or horn, giving the appearance of a 

 " Cap of Liberty." It is found frequently on the South Coast. 



29. The Tusk, or TOOTH SHELLS (Dentalium), have already been referred to. 



BIVALVES. 



i. The Oyster (Ostrea edulis) has two unequal valves, as the shells are 

 technically called, upon the left of which, being larger and more hollow, it 

 always lies. It is only to be found in places where the bottom is muddy, 

 because upon a sandy bed the oyster would be in danger of getting sand into 

 the hinges of the shells, and thus be unable to close them, in which case it would 

 quickly die of suffocation. Hence estuaries are a very favourite site for an 

 oyster bed. 



There are about a hundred different species found all over the world, most 

 of them being edible. 



The oyster spawns in May or June the " spat " or spawn resembling fine 

 slaty dust. A single oyster lays nearly 1,000,000 eggs, and she keeps them in 

 her gills (known more popularly as the " beard ") until they are hatched 

 that is, from May to July during which time oysters are " out of season." 

 Then she opens her shells, and squirts out the young ones in a cloud not unlike 

 a puff of smoke. Each young one is provided with swimming organs (cilia), 

 like fine eyelashes. Later on they sink down and fasten themselves to a rock 

 or stone, and thenceforward they never move again. They reach their full 

 size from the fifth to the seventh year. 



They obtain their food in the following way: The mouth is under the 

 " beard " or gills, and these organs not only suck out the air (which is in the 

 water), but they filter out every minute particle of decaying matter upon which 

 the oyster can feed, and thus pass it on to the mouth. 



In many places oysters are cultivated that is, grown under protection 

 for example, at the mouth of the Thames, at Whitstable, and elsewhere. When 

 protected thus they attain their full size in four years. 



It scarcely need be said that the spawn is greedily eaten by fishes, but it 

 is not so well known that the Five-fingered Starfish is the greatest enemy of 

 the largest oysters. It grasps the oyster firmly, and, when the shell opens 

 slightly, sucks out the inmate ! 



The inner lining of the shell is known as Mother-of-pearl, and if a grain 

 of sand or other foreign substance should get inside between the animal and 



