132 The Under-Water World 



to all, being cast ashore by every land- 

 ward gale. The remarkable Worm Shells 

 that inhabit the coral reefs are allied to 

 the whelks. Sooner or later the shell of 

 this animal becomes hemmed in by 

 ever growing coral. What is to happen ? 

 To prevent being walled up by the more 

 active organism the worm shell has 

 obviously to take drastic action to meet 

 the situation. It does so by ever in- 

 creasing the mouth of the shell until it 

 becomes prolonged into a twisted tube. 

 Always taking the line of least resist- 

 ance, the worm shell fashions its dwell- 

 ing to coincide with every vagary 

 of coral growth the result, a Gordian 

 Knot. 



Judging by the records of the rocks 

 the univalves are immeasurably older 

 than the bivalves. The exact origin of 

 the bivalve is a matter for conjecture, 

 but geological investigation justifies us 

 in assuming that lethargic univalves took 



