SHORES OF THE CLYDE AND FIRTH. 
103 
Slowly we see the ring diminishing at the edges and spread- 
ing out in a web towards the centre, and the spreading 
continues until the whole posterior is covered over like the 
head of a drum, except a neat little round hole in the 
centre. Here we have, in the first place, the cilia, as in 
other cases, the motive power for raising the water from 
below; next we have the tongues with their gummy threads- 
raising the larger dust particles the force of the incoming 
current is unable to lift higher than the base of the foot; 
and, lastly, we have the posterior mantle making and 
regulating the size of the orifice to admit the nett quantity 
of water the cilia are capable of lifting ; and this may be 
either when the foot is on its place upon the rock or when 
withdrawn within, as we have just observed in this case. 
If these observations are not the solution of the points, it 
may not be counted egotistical to say that they at least 
point to the leading strings. 
' The excavating habits of the pholas families have had a 
powerful influence in altering the aspect of many portions 
of sea-board. They, of course, do not individually go more 
than from three to six inches deep ; but one generation after 
another working in the same fashion, with the aid of the sea 
washing in and out, and wearing and tearing away bit by 
hit the remaining portions of the honey-combed rock, must, 
since their formation, have levelled and scattered vast beds 
of rock ; and still the excavations are going on. The boring 
habit in the young animal is early begun ; indeed, it may 
be said to be immediately after escaping from the spat, or 
egg. The smallest hole I have yet detected was about the 
size of a barley pile; but the creature itself must have been 
much less in size. How wonderful it is, then, that such a 
tiny object, provided with a boring tool as thin as a wafer, can,, 
with ease, excavate for itself a home out of the flinty rock. 
