HALF AN HOUR WITH SHELL-FISH (BIVALVES). 215 



you are more curious, at low water you may see 

 the animal itself ensconced in the habitation it has 

 made. No other mollusc has had so much written 

 about it as this. Theories innumerable have been 

 propounded to account for the way in which it 

 excavated hollows in the hardest and the softest 

 rocks alike. It has been suggested that it secreted 

 an acid, but no acid was ever found in the animal ; 

 and again, it must be a peculiar one that would 

 dissolve away all kinds of rock. Then it was con- 

 tended that the foot was covered with flinty spicules, 

 with which the Pholas managed to rasp away the 

 rock. Unfortunately, the microscope showed no 

 traces of these flinty teeth ! Here again the 

 aquarium has contributed materially to natural 

 history, for by its means the Pholas has been care- 

 fully watched, and the manner with which it mines 

 its holes is no longer a secret. To understand how 

 this is effected, it will be necessary first to attend 

 to a few points in the anatomy of the animal. The 

 external shell is white, and open at each end. Out 

 of one end proceeds a long tube or siphon, which 

 communicates with the aperture of the boring, and, 

 being contractile, can squirt out a jet of water, 

 when irritated, to some distance. At the other end 

 of the valves is the fleshy foot, which can attach 

 itself to the bottom of the tunnel, or to the rock. 

 The valves at this end are much thicker than at 

 the other, and are further strengthened by a 

 chambered structure they possess. The exterior, 



