74 THE LIFE OF THE MOLLUSCA 



admission of air during aestivation. In some species 

 this consists of a simple notch in the peristome close 

 to the suture, but in others the notch becomes con- 

 verted into a short-necked aperture placed a little way 

 back from the mouth, while in yet others a distinct 

 tube is developed, either attached to the body whorl 

 or standing free from it. 



Molluscs accustomed to hibernation or aestivation 

 probably are longer lived than their fellows ; but as 

 a matter of fact very few real data exist concerning 

 molluscan longevity, which probably varies a great 

 deal. Many of the aquatic species are annuals, some 

 of the Nudibranchs require part of two years for 

 their full development. The young of Doris and 

 Molts are born in the summer in the warm shallows 

 near the shore, drift out to deeper water, where they 

 pass the winter, and in the following spring return 

 to the tidal rocks, attain their full growth early in 

 the summer, and after spawning-time disappear. 

 The Mussel, Cockle, and Ship-worm attain their full 

 growth in a year, Pteria in two years, the Oyster in 

 five, though in Oyster-beds it may live for ten years. 

 The huge Tridacna is credited with eight years, while 

 the Swan Mussel (Anodonta) is said to reach twenty 

 or thirty, and many of the marine Gastropods 

 probably attain a comparatively great age. The 

 Periwinkle has been kept in captivity for nearly 

 twenty years. 



The Land Snails are mostly biennial, but the 



