PHOLAS CRISPATA, LINN., AS A BORER. 37 
Pholas crispata, Linn., as a Borer. By Jounn Smita. 
[Read 28th March, 1893.] 
Various theories have been advanced to account for the manner 
in which the different species of Pholas excavate their burrows. 
These theories are (1) that the shell turns round in the burrow, 
in the manner of an auger, the sharp edges and spines of the shell 
thus cutting away the rock ; (2) that sharp siliceous spicules are 
embedded in the foot of the animal; (3) that the animal secretes 
an acid which it ejects into the burrow ; and (4) that the hole is 
bored by the mere action of the bare foot on the rock or other 
substance to be excavated. 
If the first theory were true, we should certainly see indications 
of wearing on the edges of the shells and on the spines. Even the 
most highly-tempered steel cannot resist being worn when used as 
an auger with which to bore holes into rocks. But there are no 
signs of wearing on the shells of Pholas, neither is the foot provided 
with any siliceous spiculz, as the second theory would demand. If 
the holes had been excavated by means of a chemical solvent, they 
would show indications of corrosion, whereas the sides and bottom 
of every burrow are nearly smooth. The fourth and last theory 
seems to afford the most satisfactory explanation, and only requires 
a slight augmentation to render it perfect. 
A few years ago I discovered a small colony of Pholas crispata 
on the Ayrshire coast. They had bored into a reddish volcanic 
ash of carboniferous age. On digging out a few dozen specimens, I 
found that in each case, so far as specially observed, the sole of 
the foot (if I may be allowed the expression) was thickly covered 
with a layer of sand firmly fixed to it by mucous secretion. On 
observing this, I saw at once that the boring was effected by the 
animal using its foot primed with sand, in the manner that some 
savages are said to bore holes with a stick, the point of which is 
now and then wetted and dipped into sand. 
