ii2 THE MOLLUSCA 



short, free-swimming period is over, settle down on some suitable 

 spot, and remain there for the rest of their lives, existing on the 

 minute particles of animal and vegetable matter carried to their 

 mouths by the water which flows between the open valves of the 

 shell. Others are able to swim through the water by rapidly 

 opening and shutting their shells ; and some possess a well-developed 

 foot by which they can move from place to place, burrow in sand, 

 or even, in some species, in wood or hard rock. 



The Oyster is typical of the molluscs that lead an entirely 

 sedentary life. It has no foot, the edges of the mantle are free, 

 and bordered by a short fringe. The valves of the shell are unequal, 

 the animal lying in the deeper of the two, on which the flat valve 

 closes down like the lid of a box. 



The sexes in the British Oyster are not separate, each individual 

 functioning as both male and female at different periods. Whether 

 the oyster fecundates its own eggs is still a vexed question, some 

 authorities holding that the oyster is first male and then female, 

 the spermatozoa being set free before the eggs are produced ; 

 others, that the individual is first female and then male, in which 

 case the oyster fertilises its own ova. 



The eggs are retained within the parent shell until they are 

 hatched, and then expelled in clouds which look like puffs of 

 smoke in the water. Warm weather is absolutely necessary for 

 the safety of the " spat," as these young oysters are called, and 

 the spawning season lasts from about April to May. Each young 

 oyster is enclosed in a tiny transparent shell and provided with 

 a fringe of cilia with which it paddles its way through the water. 

 In about forty-eight hours' time the spat falls and adheres to 

 the rocks, shells, and stones at the sea bottom, and the young 

 oysters lose their cilia and settle down for life. 



The oyster increases in size by adding a fresh layer to the 

 margin of its shell from time to time. The new ring is at first 

 almost as thin and transparent as gold-beater's skin, but in a short 

 time it thickens and hardens into shell. The oyster is not full 

 grown and fit for table until it is five years old ; and if it 

 does not share the fate of so many of its fellows in being cut 

 off in its prime, it may live for ten years or more ; but after a 

 time it ceases to add new rings to its shell, and simply increases 

 the thickness; 



