WILD LIFE OF ORCHARD AND FIELD 



cient to resuscitate them. They survive so well 

 being buried in the ballast of ships that at almost 

 every sea-port you may find species imported in 

 that way, which came to life when the ballast was 

 dumped at the time of unloading, and very likely 

 they find their home-plants growing near equally 

 the subjects of accidental importation. 



Such are some of the methods of dispersion. 



As to the longevity of snails, little is known, 

 but some individuals no doubt attain great age. 

 Some species of Cylindrella have a habit of desert- 

 ing the point of the spire of their long, slender shells 

 as they grow old, which abandoned portion speed- 

 ily becomes dead, and cracks off upon the least 

 injury. The sign of a perfect adult shell in these 

 species, therefore, is that it is broken. Mr. Thomas 

 Bland, the distinguished student of West Indian 

 conchology, discovered this curious fact. After 

 the cylindrella has thus voluntarily left the upper 

 part of the shell, it builds a partition across behind 

 it, and other mollusks are often driven to a sim- 

 ilar expedient by accident or the decay of extreme 

 age. This is called decortication, and is almost 

 always to be seen in the beaks of the larger unios, 

 or fresh- water mussels, of our inland rivers, as well 

 as in many spiral univalves that live in swift-run- 



309 



