38 TRANSACTIONS, NATURAL HISTORY SOCIETY OF GLASGOW. 
In digging its burrow, Pholas crispata very seldom impinges 
on the excavation of its neighbour, although a dozen holes may 
be quite close together; and in this respect it differs much from 
Saxicava rugosa, which constantly runs its burrow broadside into 
another already formed. The holes formed by P. crispata are 
carried down to a depth of six or seven inches, and are very 
small in diameter at the surface, but increase gradually in width 
with the growth of the animal. 
When in the act of digging, it appears to open out its valves 
till they fit tightly into the hole, and in this position it can use 
considerable force in excavating with its foot. The fine material 
ground off at the bottom of the burrow will be easily ejected 
‘therefrom by the animal inverting the action of its siphon, which 
it can do instantly and with vigour. 
It may be asked whether the Pholas is always in a position to 
command a supply of sand wherewith to prime its foot ; and this 
question may, we think, be answered in the affirmative. The 
habitat of Pholas is between tides, or a short distance below low 
water mark; and in this zone there is always plenty of gritty 
material, owing to the constant agitation of the water. 
When P. crispata bores into rock (generally limestone, shale, 
fire-clay, or voleanic ash) it is invariably much smaller in size 
than when it bores into muddy clay, and even in glacial and raised- 
beach strata this difference in size may be observed. Boring into 
rock is evidently for it a much heavier undertaking than boring 
into clay, and in the former case it has clearly less time to “ grow 
fat” than when engaged in the latter operation. 
The light emitted by the species of Pholas has engaged the 
attention of naturalists since the time of Pliny. It must be of 
considerable service to the mollusk, by attracting small marine 
animals which get drawn into the siphon and are used as food, 
and may also possibly help to illumine its otherwise rather dark 
dwelling. 
In her Popular History of the Mollusca, Mary Roberts has 
collected a few notes on the subject of this light, which may be 
worthy of quotation. ‘There is another quality in this tribe of 
mollusks which demands a brief description. It is that of emitting 
a phosphorescent liquor, which illuminates whatever it touches. 
This peculiarity, observed by Pliny and other ancient naturalists, 
