46 THE STORY OF LIFE IN THE SEAS. 



of that mouth there is a ribbon beset with 

 numerous sharp little teeth, which by a compli- 

 cated mechanism can be worked backwards and 

 forwards in such a manner that it can bore a hole 

 through very thick and dense shells ; and, the soft 

 parts being reached, a tube is protruded which 

 dissolves and sucks them up into the animal's 

 stomach. 



Many people must have noticed that numbers 

 of the bivalve shells that are cast up on the sand 

 at low tide are perforated close to the hinge by a 

 neat little round hole. This is the hole made by 

 some predaceous Gastropod which, having killed 

 its prey and devoured all that is digestible of it, 

 leaves its empty shells at the mercy of the waves. 

 Amongst the rocks numerous species of Gastro- 

 pods are found, some undoubtedly carnivorous, 

 others herbivorous. The many beautiful forms 

 and colours that their shells assume may be seen 

 in any good museum or conchological cabinet. 

 Some of them are very minute, others are pro- 

 vided with a shell more than a foot in length ; 

 some are marked with numerous coloured spots, 

 others with bands or lines ; some have perfectly 

 smooth shells, others are ribbed or spiny. It 

 is extremely difficult to account for all these 

 modifications, partly because it is impossible to 

 study the animals alive in their natural habitats 

 a few fathoms below the surface of the sea, and 

 partly because life in the shallow waters must be 

 so complicated that we are at a loss to understand 

 the value to a species of slight modifications in 

 structure such as these. The difficulty that has 

 been found in explaining these various forms and 



