The Shell-builders. 



grips the sand firmly while the long shell is hauled 

 into the entrance of the burrow. Pointing its toe once 

 more, the Solen elongates it and dives it still further 

 down, the end of the foot suddenly swelling out until 

 it becomes firmly wedged in the shaft, while the upper 

 part contracts, dragging the shell downwards with a 

 jerk. In this way, by alternately contracting and 

 plunging its toe downwards and then expanding it, 

 the Solen excavates the long, narrow shaft, at the bottom 

 of which it spends most of its life. 



A number of bivalve Molluscs are expert masons, 

 excavating chambers in the rock in which to dwell. 

 One that is to be found on almost any rocky shore is 

 the Pholas, or " Piddock " as it is popularly called on 

 many coasts. It is a small animal, with a pair of 

 milk-white shells which are very thin and brittle, and 

 do not completely cover the Mollusc. As soon as 

 its shell is formed, the young Pholas proceeds to ex- 

 cavate for itself a rocky cave, large enough to make 

 a comfortable dwelling-place, but too small for its 

 enemies to enter. There it spends the rest of its life, 

 a willing hermit in a rock grotto, the interior of which 

 it gradually enlarges to meet the requirements of its 

 own increase in size. For many years the way in which 

 the Pholas excavated its home in the rocks was a sore 

 puzzle, and many were the theories, more or less dog- 

 matically expounded, as to how it was accomplished, 

 such as the possession of an acid corroding fluid that 

 gradually dissolved the rock away ; or that the Pholas 

 scraped away at the rock with the edges of the valves 

 of its shell, turning round and round the while like 

 a living drill. Although the valves are beset with 



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