NOTES AND QUERIES. 439 



equal in diameter to tlie greatest width of the shell, while the foot 

 alone projects farther into the cavity, would seem to show that the 

 foot only is used. In the first instance, I think that the foot used in 

 conjunction with the siphons acts as a pump, the fragments of sand- 

 stone, shale, and mud loosened by the foot being ejected by the 

 siphons and got rid of. Exactly the same process is adopted on 

 ships and boats where the pumps are close to the keel. I have seen 

 a cup-shaped hole worn in the solid oak purely by suction. The 

 objection may be raised that this is due to oxide of iron, but in my 

 experience (that is, forty years ago) wooden pumps were most commonly 

 used. Many a ship has been lost through the pump sinking into the 

 cavity so formed, and becoming choked and useless. However 

 accomplished, the power of boring possessed by Pholas is very con- 

 siderable, for I have seen a burrow bored through three inches of 

 hard ferruginous sandstone, and six inches into the shale beneath. 



Economic Uses. — Pholas was used by the fishermen at St. Andrews 

 sixty years ago as bait for God and Haddock. At that time much of 

 the shale where the mollusc lived was dug up ; not an example of 

 PJiolas is now to be seen at those parts, although these shellfishes 

 are abundant where the shale has not been disturbed. Like all other 

 living creatures, they have- to bear their part in the struggle for 

 existence ; by far the greater part of the larvae must perish through 

 failing to find a proper resting place, or by being devoured. Even the 

 adults in their burrows are not secure, for crustaceans (sessile-eyed) 

 devour them, and sponges and polyzoa grow over the mouths of their 

 burrows, preventing the ingress of food and water for respiration ; by 

 this latter occurrence many of the adult forms are killed. 



Other horing Molluscs. — Pholas is not the only mollusc which 

 makes a barrow. Tapes, Saxicava, My a, Solen, and the common 

 Limpet all construct tunnels. Solen burrows in the sand, and if dug 

 out of the burrow and laid on the surface of the sand they protrude 

 the foot and penetrate into the sand like a plough until deep enough 

 to suit them. The foot then swells and shortens, drawing the shell 

 down after it. The foot is again extended downwards, a new grip is 

 taken, and the process is repeated until the creature is finally pulled 

 downwards out of sight. Such is the Solen' s method of escape. Mya 

 bores through clay in exactly the same manner. 



The smallest Pholas I ever found in the shale was about the size 

 of a hemp-seed, and penetrated about one inch. The hole on the 

 outside was only a hair's-breadth in diameter. The Pholas itself 

 was in a small cavity. As it continues boring it makes the hole 

 larger until it gets down about six or eight inches. When the Pholas 



