74 ‘THE LIFE OF THE MOLLUSCA 
admission of air during zstivation. 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 estivation 
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 
folis 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, Ptevia 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 
