20 C. M. YONGE 



life, the number of threads and strength of attachment are far less 

 than in Botula (Yonge, 1955). This condition is correlated with an 

 initial chemical softening of the rock. The mantle tissues protrude 

 from between the valves anteriorlv, spreading out laterally over 

 their surfaces. When the animal is pushed against the head of the 

 boring, these tissues are applied to the surface of the rock ( Fig. 10 ) . 



Fig. 10. Lifhophaga phimula, boring opened from above to show anterior 

 end of shell with mantle tissues protruded against head of boring. Arrows 

 show direction of ciliarv currents. (From Yonge, 1955.) 



Although the presence of acid, as such, cannot be demonstrated, 

 softening and dissolution of calcareous rock against which the tis- 

 sues are applied has been demonstrated by Kiihnelt ( 1930 ) , Yonge 

 (1955), and Hodgkin (1962). The last named has also demon- 

 strated experimentally that, when placed in a suitable-sized bore 

 hole in mudstone, L. plumula is unable to enlarge it, although it 

 can certainly rotate within it. It is quite certain that Lithophaga 

 cannot bore without some initial chemical softening of the rock. Its 

 own shell is protected against chemical action by the thick, and in- 

 variably unbroken, layer of periostracum. 



The glands responsible for the softening, probably acid-contain- 

 ing, secretion are situated within the middle mantle fold immedi- 

 ately anterior and posterior to the ligament. Similar glands also oc- 

 cur in Botula and in all Mytilidae. In these mussels they secrete 

 mucus, which entangles particles that fall on these regions (es- 



