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 orifice and con- 

 veyed in a current through 

 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 cleansing 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 

 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 dactylus 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 

 Email compared with the size of the vermiform body, and are 



