How Animals Work. 



inequalities producing a rasp-like structure, it is chiefly 

 by means of its foot, the base of which is furnished 

 with a layer of renewable sharp flinty crystals, that the 

 Pholas excavates its cave, but the exact manner in 

 which it is accomplished is still imperfectly under- 

 stood. 



Saxicava is another bivalve mason, excavating tun- 

 nels often six inches in length, so that limestone rocks 

 on the coast are sometimes riddled with the borings of 

 this small, thin-shelled bivalve. It by no means con- 

 fines its attention to the rocks and reefs unfortunately, 

 but will also make its home in the base of stonework 

 piers and breakwaters, boring into the concrete cement 

 and stone used for building such structures. 



The Date-shell (Lithodomus), related to the edible 

 mussel, makes excavations in corals and hard lime- 

 stone rocks. At Puteoli, on the shores of the Mediter- 

 ranean Sea, stand the ruins of the temple of Serapis. 

 Three erect white marble columns are still standing, 

 and these are perforated at a height of nearly twenty 

 feet above the present sea-level with the excavations of 

 the Date-shell. Similar borings at the same height 

 are to be seen in the face of the cliffs near by, and 

 afford most striking evidence of the changes which 

 have taken place in the level of the land within his- 

 torical times. The temple, originally built on dry 

 land, must with the cliffs have been submerged to a 

 depth of over thirteen feet beneath the surface of the 

 sea for an appreciable period of time, during which the 

 Date-shells made their excavations, and then gradu- 

 ally the land was once more raised to its present level. 



42 



