MODE OF BURROWING. 57 



we see by the specimens before us that such a destruc- 

 tive action does in time take place, for some of these 

 have the rasping points much more worn than others, 

 many of the older ones being nearly smooth. 



The animal turns in its burrow from side to side 

 when at work, adhering to the interior by the foot, and 

 therefore only partially rotating to and fro. The sub- 

 stance is abraded in the form of fine powder, which is 

 periodically ejected from the mouth of the hole by the 

 contraction of the branchial siphon ; a good deal of the 

 more impalpable portions being deposited by the cur- 

 rent as it proceeds, and lodging as a soft mud between 

 the valves and the stone. Mr. Hudson, 1 who watched 

 some Pholades at work in a tide- pool in the chalk, 

 observed the periodic ejection of the cloud of chalk- 

 powder, and noticed the heaps of the same material de- 

 posited around the mouth of each burrow. The dis- 

 charges were made with no regularity as to time. Mrs. 

 Merrifield 2 records a curious fact. "A lady, watch- 

 ing the operations of some Pholades which were at 

 work in a basin of sea-water, perceived that two of 

 them were boring at such an angle that their tunnels 

 would meet. Curious to ascertain what they would 

 do in this case, she continued her observations, and 

 found that the larger and stronger Pholas bored straight 



1 Zoologist, p. 7819. 2 S'at. Hist, of Brighton, p. 185. 



