1 68 THE BIOLOGY OF THE SEA-SHORE 



between tide-marks to wooden piers and similar structures 

 is enormous, while the rock-boring forms play a considerable 

 part in aiding coast erosion. We shall consider these boring 

 forms under two headings, (a) rock-boring forms, (b) wood- 

 boring forms. 



(a) Rock-boring Forms. — The most important of these 

 are the two bivalves : the Piddock {Pholas dactylus) and the 

 form known as Saxicava rugosa. Considerable difference 

 of opinion has existed in the past as to whether the boring 

 was the result of chemical or of mechanical action or of a 

 combination of the two. According to Caiman (1919), there 

 can be no doubt that the boring of Pholas " is effected mainly, 

 if not exclusively, by the rasping action of the shell, which 

 is provided with rows of spines or teeth towards the front 

 edge." The appearance of the teeth varies according to 

 the nature of the material in which the animal is normally 

 operating, being sharp and slender in specimens from chalk 

 or peat and blunt and rounded in those from harder rocks. 

 The variety of substances in which Pholas has been observed 

 to bore is said to preclude chemical action (Caiman, loc. cit.). 

 According to the same writer, the boring movements " con- 

 sist chiefly in a twisting or rocking movement of the whole 

 shell on the fulcrum formed by the sucker-like foot, the 

 muscles from which are attached to the inner blades of the 

 valves." The method employed by Saxicava in boring 

 calcareous rocks is less apparent. Caiman considers that, 

 in spite of the fact that so far no acid secretion has been 

 demonstrated nor the presence of any special gland, it is 

 certain that the process of boring is at least partly chemical, 

 since this mollusc occurs in limestone rocks so hard and 

 compact that mechanical action seems out of the question 

 for so fragile a shell. Polychset worms of the genus Poly dor a 

 are found burrowing in calcareous rocks and shells on British 

 shores and sometimes in shales and sandstone as well. 

 Caiman considers that although there may be some abrasion 

 of the rock by the bristles, chemical action must be regarded 

 as indispensable. 



Oyster shells pitted by the boring sponge Cliona celata 



