SKELETONS, SHELTERS AND SPECIAL DEFENCES 



659 



It is among lamellibranchs that the largest and most efficient rock-borers 

 are encountered. The family Pholadidae includes a series of forms that live 

 in various kinds of soft rocks, shell and wood. The piddock Pholas dactylus 

 is a rather large species which sometimes reaches a length of 15 cm and 

 makes burrows up to 30 cm long (Fig. 15.14). It bores indifferently into a 



: • '• ; y" ••• 



(d) 



Fig. 15.14. Boring Lamellibranchs 



(a) Teredo navalis in wood (x |), remainder in stone; (b) Gastrochaena cuneiformis 

 (x f); (c) Lithophaga cumingiana (x \); (d) Pholas dactylus (x f). 1: exhalant 

 siphon; 2: inhalant siphon; 3: foot; 4: calcareous lining; 5: byssus; 6: shell; 7: pallet. 

 (Partly after Otter (98).) 



wide variety of rocks — limestone, shale, sandstone, mica-schist — and oc- 

 casionally peat and wood. The piddock attaches itself at the head of its 

 burrow by means of a sucker-like foot. The valves are stout and bear 

 spines on the external surface. There is no hinge ligament, the hinge takes 

 the form of a ball joint and the two valves rock on this joint as a fulcrum 

 through alternate contractions of two adductor muscles. The piddock 

 bores by raspmg action of its shell, the spines gradually cutting away the 



