22 C. M. YONGE 



a basic sessile or nestling habit which docs, where conditions are 

 suitable, lead to boring. 



Really specialized obligatory borers include certain sipunculids 

 and polvchaetes, the carnivorous shell borers including the turbel- 

 larian Psemlosti/Iachus and the prosobranch gastropods (reiving 

 wholly or in part on chemical agencies), the cirriped Lithotrija, 

 and most of the bivalve borers. In the last-named group, where we 

 have fuller information than about an\' other group apart from the 

 carnivorous gastropods, boring is primarily mechanical by means of 

 the valves. The majority grind by lateral and forward movements 

 caused by pressure of water in the mantle cavity aided to greater 

 or lesser extent by the opening thrust of the ligament. In the 

 highly specialized Adesmacea, boring is due to the rocking action 

 of the valves on the median fulcrum of the hinge region caused by 

 the alternate contraction of the anterior and posterior adductor 

 muscles. The ligament is greatlv reduced or lost. In the boring 

 Tridacnidae, where the ligament is not reduced, the animal grinds 

 its way downward by the alternate contraction of the pedal (or 

 byssal) retractor muscles. This causes rocking in the longitudinal 

 plane on the fulcrum of the massive byssal attachment. In the bor- 

 ing Mytilidae, contraction of the posterior byssal muscles causes 

 the anterior end of the shell to bear against the head of the boring, 

 while lateral widening must be due to the opening thrust of the 

 powerful, secondarily extended ligament. But in Lithopliago, con- 

 fined to calcareous rocks, the means of mechanical boring are re- 

 duced, with accompanying modification of preexisting mantle 

 glands to supply the means for chemical softening of the rock. But, 

 as with other boring organisms, apart from the carnivorous gastro- 

 pods, we lack knowledge about the precise manner in which the 

 rock is softened although this presumably involves acid production. 



Protection has been revealed as the major biological advantage 

 of rock boring. Where borers are numerous, notably on coral reefs, 

 they may be major agents of erosion. Reefs are maintained only 

 by the exceptional powers of calcification possessed by hermatypic 

 (reef-building) corals. Fungi, sponges, flatworms, polychaetes, and 

 drills are all major pests of ovsters. On the other hand, unlike the 



