THE PHOLAS DACTYLUS. 301 
be specially guarded against the danger of suffocation. The 
interior of these siphonal canals is lined with innumerable 
vibratory cilia, by the 
action of which the water 
is drawn towards the 
branchial orffice and con- 
veyed inacurrent through 7 
the canal over the surface 
of the gills; then, having 
been deprived of its oxy- 
gen, it is expelled by a 
similar mechanism through the other tube; and it is by the 
force of this anal current that the passage is kept free from the 
deposit of mud or other substances, which would otherwise soon 
choke it up. The cleaning action of the anal current is as- 
sisted by the faculty the burrowing molluscs possess of elonga- 
ting and contracting their siphons, and the degree to which this 
may be accomplished depends on the depth of the cavity which 
the species is accustomed to make. Yet since many particles 
of matter float even in clear water, which from their form or 
other qualities might be injurious to the delicate tissue of the 
viscera to be traversed, how is the entrance of these to be 
guarded against in an indiscriminating current? A beautiful 
Donax. 
a, b. Si; hons. 
contrivance is provided for this necessity. The margin of the 
branchial siphon, and sometimes, though more rarely, of the anal 
one, is set round with a number of short tentacular processes, 
endowed with an ‘exquisite sensibility and expanding like 
feathery leaves. In Pholas ductylus this apparatus, which is 
here confined to the oral tube, is of peculiar beauty, forming a 
network of exquisite tracery, through the interstices or meshes 
of which the water freely percolates, while they exclude all 
except the most minute floating atoms of extraneous matter. 
Thus admirably has the health and comfort of the lowly shell- 
fish been provided for that spend their whole life buried in 
sepulchres of stone or sand. 
The fragile shell of the pholades seems to have prompted 
them to seek a better protection in the hard rock: a similar 
necessity may have induced the shipworm to drill a dwelling 
in wood. Its shells, which are only a few lines broad, are very 
small compared with the size of the vermiform body, and are 
