302 DIMYARIA.—PHOLADIDA. 
interior of these tubes is said to be lined with 
innumerable delicate cilia; by the action of which 
the surrounding water is drawn towards the enter- 
ing orifice, and conveyed in a strong current 
through the tube over the surface of the gills. 
Then, having been deprived of its oxygen, it is 
poured through the other tube and expelled in a 
jet at its extremity, by a similar machinery. 
This apparatus of double siphonal tubes is prin- 
cipally developed in those species which burrow, 
whether in sand, mud, wood or stone. As the 
burrowing bivalve usually, if not always, dwells 
in the interior of the passage it has excavated, it 
is needful that there should be a communication 
with the external water, and hence a hole is always 
found extending to the surface of the material 
bored. ‘The entering and departing currents keep 
this passage clear, a process which in mud or sand 
might seem at first not very easy of accomplish- 
ment. It is facilitated, however, by the faculty 
which the boring bivalves have of lengthening the 
siphonal tubes at will; and the degree to which 
this may be accomplished depends on the depth of 
the cavity which the species is accustomed to 
make. 
If we take one of the stone-boring Mollusca, a 
Pholas or a Saxicava for example, from its excava- 
tion, without injuring the animal, and place it in 
a glass vessel of sea-water, it will not be difficult 
to detect the currents in question, even with the 
naked eye; though a lens of moderate power will 
render them more distinctly appreciable. The 
vessel should be so placed as that the light may 
be nearly, but not exactly, opposite to the eye. 
By this arrangement the minute atoms of floating 
