ROCK-BORING ORGANISMS 17 



lions of unicellular algae or zooxanthellae. The Tridacnidae literally 

 "farm" plants in these illuminated tissues, and this is the biological 

 explanation for the remarkable change in form (Yonge, 1936, 

 1953). The presence of the hinge, instead of the free margin of 

 the valves, on the underside alongside the byssus has made boring 

 possible. Running between the bvssus and the inner surface of the 

 valves are extremely powerful pedal (or byssal) retractor muscles. 

 Alternate contraction of these muscles causes the animal to rock in 

 the longitudinal axis on the fulcrum of the byssus. These rocking 

 movements are totallv distinct from those of the Pholadidae, which 

 are in the median transverse plane and caused by alternate con- 

 tractions of the two adductors. 



Entry in T. crocea is vertical but to some extent diagonal to allow 

 for undercutting of the byssal attachment on the one side while 

 new threads are formed on the other. The byssus is attached to a 

 pillar ( Fig. 7, R ) at the base of the boring which fits into the byssal 

 gape on the underside of the shell. The thick-ridged shell valves are 

 always greatly eroded where thev bear against the surface of the 

 rock. After they begin to bore (when about 1.4 cm long), the 

 animals must quickly penetrate to the depth of the shell valves, 

 which remain flush with the surface during growth to a final length 

 of some 10 cm, when the boring will be about 7 cm deep. The 

 animal is imprisoned within the boring, the length of the shell be- 

 ing greater than that of the opening (see Fig. 7). Except when 

 withdrawn during exposure at low tide, the hypertrophied siphonal 

 tissues spread out over the surface of the rock, completely covering 

 both the margins of the valves and the sides of the boring. The 

 high temperature at which these animals spawn, about 30 °C, con- 

 fines them to shallow water in the mid-tropics. 



vii. SuPERFAMiLY Mytilacea. Family Mytilidae. This large 

 family, which includes the common mussels, contains two important 

 genera of rock borers, Botula and Lithophaga. Both are byssally at- 

 tached throughout life, but whereas the former bores only into soft, 

 usually noncalcareous, rocks such as mudstones and shales, the 

 latter is confined to calcareous rocks — limestones, calcareous shales, 



