200 BULLETIN OF THE BUREAU OF FISHERIES. 
MODE OF BURROWING. 
The manner of mechanical formation of the burrow has been one of the most 
debated questions in the natural history of the ship-worms and their allies. In 
the former this work has been assigned by various observers to various structures. 
In some cases it was thought some chemical solvent assisted the mechanical action 
by softening the wood. Hancock thought there were siliceous particles in the 
mantle to do the work. Quatrefages thought it to be the “cephalic hood” (a 
thickened modification of the mantle at the anterior end on the dorsal side), aided 
by some chemical solvent. This structure he described as muscular, though it is 
but little muscular and could do no such important work as he assigned to it. 
Jeffreys ascribes this function to the foot, which by other observers was described 
as wholly absent. Osler, in 1826, had suspected the real mode of formation, though 
he did not actually observe it. 
The shell is the agent, assisted by the foot, as I have actually observed in young 
specimens under the microscope; and the whole structure of the shell and the 
arrangement of its adductor muscles confirm this observation. The teeth on the 
anterior edges of the valves point outward and backward. On both dorsal and 
ventral portions of each shell valve (fig. 18 and 19) there are stout calcareous knobs 
which form a double pivot by which the valves are swung upon each other by the 
contraction of the adductor muscles. The mode of burrowing is as follows: While 
the foot performs a cupping action, thus drawing the shell close against the surface 
of the wood, the powerful posterior adductor muscle contracts, so that the teeth on 
the shell rasp away the wood. The valves are brought to the original position by 
the small anterior adductor. The comparatively very large posterior adductor is 
therefore the active agent that does the work, aided by the foot; the shell with its 
teeth is the tool with which the work is done. 
INGESTION OF WOOD AND FOOD. 
As in other lamellibranchs, a constant stream of water is passing through the 
siphons when they are extended from the outer end of the burrow into the water, 
and this serves for respiration and also contains the small organisms which serve 
for food. These consist mainly of diatoms and simple floating alge, with other 
minute organisms. Small crustacea and other animal forms seem to be almost 
never eaten. 
The particles of wood that are rasped away in forming the burrow are ingested, 
as the only means of getting rid of them. It has often been debated whether they 
undergo any digestion in the alimentary canal, and I am inclined to think that they 
contribute something to the nutrition of shipworms. Boring I believe to be a 
periodical function, perhaps alternating with more active ingestion of food. This 
is indicated by two facts. In the first place, it seems that while the new-formed 
teeth are being cemented to the anterior edges of the valves the shell could not be 
used. Also, the ccum of the stomach contains almost wholly particles of wood, 
which indicates that while the animal is burrowing, the orifice into the czxcum is 
open. The very large fold of the internal mucous membrane of the cecum (fig. 
29-31) seems so eminently constructed for absorption that I think there must be 
some digestive action on the particles of wood within it. 
