THE BORING HA BITS O F THE PHOLAS. 



JOHN IRVING, M.D., 

 Scarborough 



Mr. W. Harrison Hutton's experiments with pholades, as 

 related in the D^cembsr issue of ' The Naturahs / p. 423, are 

 of interest as confirmatory evidence of observations made by 

 other marine workers. 



Occasionally some naturalists slill obtiude an ancient 

 untenable theory, that these molluscs secrete an acid to facilitate 

 boring operations, regardless of the fact that any acid, capable of 

 chemically acting on stone, would destroy the shells and tissues of 

 borers. Othei theorists lay too much stress on the strength, con- 

 formation, and structural peculiarities of shells, and pay too little 

 attention to the erosive effect of sea-water continuously applied. 



Mr. Hutton indicates the process as a simple one, due to 

 shell attrition under the influence of water, and rightly con- 

 cludes that it begins at a very early stage, when the infant 

 bivalves, fortuitously or otherwise, find their niches in depres- 

 sions, or irregularities, or crevices in the rocks. These tiny 

 pits, which temporarily accommodate the young pholades, are 

 the softer parts of the rock already being excavated by the :-ea. 

 They are often lined by a film of sticky mud to which the 

 youngsters, almost microscopic in size, easily adhere, and by 

 which they are practically glued into position as the mud 

 increases through continued rock disintegration. Slowly but 

 surely they gravitate, foot downwards, until they are half 

 buried in semi-liquid material, when rotary movements com- 

 mence to shape their destiny. Their shells, owing to their 

 extreme fragility at this stage, are of minor importance com- 

 pared with the aqueous element. All pholades bore perpen- 

 dicularh^ All entrance apertures remain small, yet the bore 

 gradually and regularly increases in calibre, from above down- 

 wards, in exact correspondence with the growth of the animal. 



Those who have frequent opportunities of examining stone- 

 borers and their excavations, know full well that, barring 

 accidents, they are life tenants of their holes. There are no 

 exits for them. The adult pholas, outside its retreat, is a 

 helpless being. Without a point d'appui it cannot bore. 

 Thrust into an artificial cavity which keeps it erect, foot down- 

 wards, the difficulty is partly overcome, but this never occurs 

 in the sea, and so the ill-fated mollusc that bores through a 

 rock, too thin for its purpose, becomes food for the crabs. 

 The sine qua non of a pholas hole is an aperture just sufficient 

 to permit extrusion of siphuncles into water above, and prevent 

 entrance of undesirable guests, and a base whose greatest 

 diameter perfectly agrees with the greatest width of shells. 

 Pholades, therefore, do not bore beyond their extended length, 

 nor in calibre beyond the size of their shells ; to do so would, 

 be to court disaster. 



1912 Feb. I. 



