150 THE SEAS 



made of calcareous matter but this is protected from the 

 action of the acid, as we have seen, by the covering of horny 

 matter. The acid-producing gland is not present in closely 

 related mussels which do not bore into rock. The borings 

 of the Date-mussel have proved of great value in a somewhat 

 unexpected connection, that of earth movements. The 

 classic example is the borings in the limestone pillars of the 

 Temple of Seraphis at Pozzuoli near Naples. These pillars 

 are to-day some distance above sea level, but for some 

 distance upwards are perforated with the burrows of the 

 Date-mussel. Since the temple must originally have been 

 built above sea level, it is clear that the land sank until 

 the pillars were covered at least to the extent to which 

 they have been burrowed into. It is just as obvious that 

 since that time the land has risen again so that the temple 

 is once more on dry land. Thus the presence of the holes 

 made by the Date-mussel enables us to prove that the 

 land in the neighbourhood of Pozzuoli has both sunk and 

 risen again within historic time. 



Crustaceans do far less damage to stone than to wood. 

 Both limestone and coral are bored into to a slight extent 

 by certain barnacles, but the only Crustacean which does 

 much damage is another species of the Sphseroma which 

 bores in wood. This creature can only work in soft sand- 

 stone or claystone but has done considerable damage in 

 New Zealand by destroying harbour works made of the 

 latter material. 



The rock-boring animals cannot obtain any food from 

 the rock ; they make their burrows and live in them 

 purely on account of the shelter with which they are pro- 

 vided. The borrows are never long and the animals are 

 in free commimication with the water outside and from 

 which they take their food which in all cases consists of 

 fine particles or microscopic animals and plants. In general. 



