BORING LIFE 149 



rarer rock-boring molluscs of which perhaps the most 

 interesting is Petricola pholadiformis which, though it 

 belongs to a family of bivalves far removed from the 

 Piddocks, has come, as a result of its similar habit of 

 life, to resemble these animals to a striking extent. The 

 commonest of all rock borers is a smaller mollusc, never 

 more than about one inch long, called Saxicava (Plate 55) 

 which is ever^^where abundant both in deep and in shallow 

 waters. Usually it is found in limestone rocks, often in 

 those of such hardness that it seems impossible that so 

 comparatively delicate a shell can have excavated the 

 burrows. It is probable that at least a certain amount 

 of the boring is performed by chemical means, although 

 this animal is also found in rocks, such as sandstone, where 

 chemical action would be unavailing. No especial glands 

 for the production of acids for eating away the rock have 

 been discovered in either Saxicava or the Piddocks but it 

 may be that the soft parts of the body which project 

 beyond the shell may exercise a solvent action of some kind. 

 There is one rock borer which undoubtedly makes its 

 burrows by the aid of chemicals. This is the Date-mussel 

 of the Mediterranean and tropical seas, so called because 

 of its resemblance in colour, size and shape, to the date. 

 It is a close relative of the common mussel. The brown, 

 date-like colour of this animal is due to the presence of a 

 thick homy layer over the surface of the shell, which is itself 

 very fragile and devoid of teeth or spines. It always bur- 

 rows in limestone or some other calcareous rock (Plate 56) , 

 not by the action of the delicate shell but by the aid of an 

 acid which is produced by a special gland in the soft 

 tissues just A^ithin the shell. Acid attacks calcareous 

 matter very readily but not other forms of rock and this 

 is the explanation of the invariable preference of the Date- 

 mussel for calcareous rocks. The shell, of course, is alsa 



