ROCK-BORING ORGANISMS 



13 



Fig. 4. A, Petricola carditoides; B, P. pholadiformis; showing greater 

 specialization of the shell for boring in the latter. (From Yonge, 1958.) 



evidence that the boring habit is facultative and not obHgatory. In 

 his account of Hiatella gallicana and H. arctica, Hunter (1949) 

 states that the habit of the adult is determined by the nature of 

 the substrate on which the larvae settle: if it be the surface of 

 a soft homogeneous rock they will bore, if it be a hard but creviced 

 rock surface they will become byssally attached. The byssus threads 

 (but not the gland) are lost in animals which bore, the process 

 being entirely mechanical (the valves becoming much eroded) by 

 means of water pressure forcing the shell valves apart (Fig. 5). 

 Hunter points out that "the boring movements evolved with little 



