ROCK-BORING ORGANISMS 

 DA 



11 



Fig. 3. Pholadidea peniUi, left valve, showing dorsal {DA) and ventral 

 (VA) articulations, anterior {AA) and posterior {PA) adductors, and apophy- 

 sis {AP) to which retractor muscles of foot are attached. Foot protrudes 

 through pedal gape anteroventrallv. Shell valves rock on axis of motion indi- 

 cated by broken line. (After Lloyd, 1897.) 



valves. The changed position, and function, of the foot involves 

 change in disposition of the pedal muscles. These are now attached 

 to a pair of bladelike shell processes which extend ventrallv from 

 within the hinge region. These apophvses (Fig. 3, AP) are found 

 only in the Adesmacea. 



During boring the animal rotates, the attaching foot moving first 

 in one direction, then in the other. Thus a smooth boring is cut by 

 the rocking of the valves on the median fulcrum. The effective 

 abrasive action is performed bv the anterior halves of the valves, 

 which are pulled apart bv the action of the posterior adductors. In 

 section the boring is round with no trace of dorsal or ventral ridges. 



In the most specialized of the adesmacean rock borers, Pholadidea 

 and Parapholas, the animal excavates a boring of a definite size and 

 then the foot with its musculature atrophies and the anteroventral 

 gape is filled in by inward growth of the mantle margins, which 

 secrete a thin callum over the entire surface. This is not true of 

 Zirphaea, Pholas, or Barnea. 



Apart from the stiff clay in which species of Zirphaea occur, to 

 depths of up to 50 cm, the pholads bore into a wide variety of sub- 



