100 
ANJMAL LIFE ON THE 
Dr. Carpenter, Philip Henry Gosse, and others, explain it in 
this fashion : " The syphon being distended with water, the 
animal sud lenly contracts it, and thus a jet is produced 
through the anterior orifice, which' washes out the part of 
the cavity occupied by the animal ; but as many of the 
particles expelled by it are deposited before they reach the 
mouth of the hole, the passage is found to be lined nearer 
its entrance by a soft mud." Harper, again, who declares 
himself to have been in a position to speak with some degree 
of certainty, says : — " During the period in which the pholas 
was actually engaged in boring, it continually threw out 
from its smallest syphon fine threads of apparently pulverised 
rock. These produced a very strange effect. Had my 
favourite been totally embedded, I should naturally have 
supposed, as Mr. Ostler does, that in order to free itself from 
the soft particles that eluded its orifice, it would contract 
its syphon, then suddenly extend it, at the same time blowing 
away the debris, and thus clearing the cavity. But such 
could not possibly be the case in th's specimen, for the rock 
was shaped so that, as I have before stated, the whole of the 
syphons and part of the valves were always exposed, and 
never at any time entirely covered, even with the softened 
particles produced by the boring operation. This phenome- 
non, therefore, evidently admit of no other explanation 
than that there must be some pedal opening through which 
the creature draws in whatever collects at the base of the 
cavity, and expels it as above-mentioned, and this without 
any filtration whatever; but," he adds, "I candidly confess 
that I have never been able to discover any pedal orifice." 
The first theory might stand good were the creatures 
always working in hard clay or soft chalky rock, where the 
entrance is ahvays wider than required ; but, I fear, it is 
impracticable in the hard rock. In a piece of stone in my 
